How to Identify Errors in a CMS Case

Important: Do Not Turn a Hypothesis Into an Allegation

A duplicate record, matching error, migration problem or deliberate false statement may be possible explanations, but they should not be stated as established fact without evidence. Ask CMS for the underlying record, source, chronology and calculation, then compare it against independent documents. This makes the challenge stronger and avoids allowing CMS to dismiss the issue as speculation.

Important: A SAR Is Evidence-Gathering, Not an Appeal

A Subject Access Request can help reveal what DWP holds, but it does not automatically stop collection, enforcement, Mandatory Reconsideration or tribunal deadlines. If a decision is challengeable, protect the statutory challenge route while obtaining the records.

Identifying Errors in a CMS case

1. VME / HMRC bulk matching

This image explains a simplified version of how CMS can obtain income information from HMRC through a secure data-matching process (often referred to as VME or a similar matching service).

It shows:

• CMS sends many requests together (a bulk request).
• The request contains identifying information (such as NI number, date of birth, name).
• HMRC matches those identifiers against its records.
• HMRC returns income information.
• CMS attaches the returned data to each case.

The second half of the diagram illustrates a hypothetical failure where:

• records become misaligned,
• identifiers are mismatched,
• information intended for one person becomes linked to another case.

This is presented as one possible failure mode, not evidence that it routinely happens.

2. Other ways CMS information can become wrong

This image groups common categories of administrative error, including:

• incorrect identity information
• duplicate records
• outdated information
• incomplete information
• incorrect relationship details
• timing problems
• data quality issues
• third-party system problems

It also shows the consequences:

• wrong maintenance calculations
• overpayments
• underpayments
• appeals
• enforcement errors

These are recognised categories of administrative error that can potentially occur in many large government systems.

3. Old CSA cases and missing payment history

This image contains two separate scenarios.

Upper section

Shows a hypothetical example where:

• an old Child Support Agency (CSA) case exists
• historical data is migrated or referenced
• payment history does not accompany the case
• CMS calculates arrears because previous payments appear absent

This illustrates one possible explanation for an apparent arrears issue but is not proof that this is what happened in any particular case.

Shows another hypothetical example:

• payments were made
• payment records become missing or disconnected
• CMS finds no payment history
• arrears are calculated from zero

Again, this is illustrating a possible administrative failure rather than stating that records are commonly deleted.

4. Full map of CMS errors

This is essentially a "mind map."

It organises errors into sections:

identity matching, HMRC data, VME exchanges, payment recording, calculations, adjustments, document management, business processes, staff error, external systems

It then links those errors to outcomes such as

• false arrears
• wrong liability
• missing payments
• incorrect assessments
• appeals

enforcement

It is intended as a conceptual overview showing where errors might occur during the life of a CMS case.

5. Receiving Parent giving incorrect information

This image illustrates a hypothetical situation where a receiving parent knowingly provides incorrect information to CMS.

Examples shown include:

• overstating the paying parent's income
• understating shared care
• claiming fewer overnight stays than actually occur

The image then shows how those claims could result in

• a higher maintenance calculation
• higher payments
• disputes

It's important to note that CMS does not rely solely on what one parent says about income. Current income figures are generally obtained from HMRC records, although information about shared care arrangements may depend on evidence from the parties if disputed.

6. Variable income calculation

These final two graphics explain the weekly income calculation used by CMS.

The diagrams show:

Annual income

divide by 365

multiply by 7

weekly gross income

deduct allowable pension contributions

maintenance calculation

The examples compare:

Regular salary

where earnings remain similar throughout the year.

The annual average tends to represent current earnings reasonably well.

7. The CMS formula Irregular income current earning capacity

CMS generally uses a historical annual income figure (normally supplied by HMRC) to calculate weekly income.

The calculation shown is:

Annual gross taxable income

Weekly gross income

Then:

Allowable pension contributions are deducted from the weekly figure.

That weekly figure is then used to calculate child maintenance.

Why this works well for salaried employees

Suppose someone earns:

£600 every week

for the whole year.

Their annual income is

£31,200

CMS calculates

£31,200 ÷ 365 × 7

= about £598 per week

That is very close to what they actually earn every week.

So averaging works well.

What the five examples are trying to show

The examples are illustrating increasingly uneven income patterns.

Example 1

Small fluctuations.

The average stays reasonably close to reality.

Little difference.

Example 2

Larger ups and downs.

The average starts moving away from what someone earns most weeks.

Example 3

Large income spikes.

Three very good months increase the whole year's average.

If income later falls, the average remains high until HMRC supplies a newer tax year.

Example 4

Very large spikes.

Perhaps someone had:

• one large contract
• one business sale
• unusually high commission

The annual average becomes much higher than current income.

Example 5

Extreme example.

Perhaps:

• sold a business
• received a one-off bonus
• completed one major project

That year's taxable income becomes extremely high.

CMS averages it across the year.

If business has now disappeared, the weekly figure may be much higher than current earnings.

What the graph means

The green line represents actual earnings.

It rises sharply.

Then falls.

Then rises again.

CMS ignores those peaks and troughs.

(should ignored the peaks but if payslips used at at those peaks they cause a problem)

Instead it draws a straight average line across the year.

That straight line becomes the weekly income used for maintenance.

It is worth noting that describing this as "false income" is not technically correct. The calculation is based on actual historical taxable income, averaged over the relevant period. The concern being illustrated is that the average may not reflect someone's current earning capacity, particularly if their income has fallen significantly after high periods.

8. CMS uses an older HMRC tax year

The infographic is based on a scenario where CMS uses an older HMRC tax year because more recent HMRC income data is not available. One important clarification is that the image overstates the position. It says CMS can choose "any one of the six years" whenever it wishes. That is not how the legislation is intended to work. In practice, CMS normally uses the income information that HMRC provides under the statutory scheme, and if the latest tax year's information is unavailable there are rules governing which earlier information may be used. It is not simply an unrestricted choice.

That said, the image illustrates a concern that can arise if an assessment is based on an older, higher-income year.

Step 1 – CMS requests income from HMRC

When a maintenance calculation is started or reviewed, CMS asks HMRC for the paying parent's taxable income.

Normally this comes from:

• Self Assessment
• PAYE
• Real Time Information (RTI)
• other HMRC records

CMS usually bases the assessment on the income HMRC supplies.

Step 2 – More recent income is unavailable

Sometimes HMRC may not yet have processed the latest tax return or current year's information.

Possible reasons include:

• tax return not processed yet
• filing deadlines
• processing delays
• corrections still outstanding

In those situations, CMS may have to rely on the income information that is available under the applicable rules.

Step 3 – Older income may be much higher

For example:

Tax Year Taxable income

2018/19 £120,000

2019/20 £110,000

2020/21 £95,000

2021/22 £80,000

2022/23 £45,000

2023/24 £28,000

Suppose the person's business has steadily declined.

Their actual income today might only be around £28,000.

If an older year with a much higher income is used because that is the information available, the resulting maintenance assessment may be considerably higher than if more recent verified income were available.

Step 4 – Weekly income is calculated

CMS converts annual taxable income into weekly income by:

Annual taxable income

Weekly gross income

Then allowable pension contributions are deducted before the maintenance calculation is made.

Step 5 – Effect of using an older higher-income year

If the income used reflects a period when earnings were much higher than they are now, the assessment may not reflect the person's current financial circumstances.

This could mean:

• maintenance payments higher than current earnings might support
• increased risk of arrears if payments become unaffordable
• disputes and requests for reassessment

Whether this is the correct legal outcome depends on the applicable CMS rules and whether there is a basis for using more up-to-date income or requesting a variation or supersession.

Step 6 – Evidence someone could provide

If a person believes the income being used is no longer representative, evidence might include:

• latest Self Assessment return
• HMRC tax calculation
• recent company accounts
• management accounts
• bank statements
• invoices
• profit and loss reports
• accountant's letter
• evidence of loss of contracts
• evidence of illness affecting earnings

Questions someone could ask CMS

Rather than assuming an error, it is often helpful to ask specific factual questions, such as:

"Which HMRC tax year was used for my calculation?"

"What income figure was supplied by HMRC?"

"Please explain why that tax year was used."

"Have you requested more recent HMRC information?"

"If more recent information was unavailable, what evidence supports using the earlier figure?"

"Please provide the calculation showing how my weekly gross income was derived."

When could this become an error?

It may become an issue if, for example:

• a more recent verified HMRC figure was available but not used;
• the wrong tax year was selected contrary to the applicable rules;
• the HMRC data was incorrect or mismatched; or
• there had been a qualifying change in circumstances that should have been taken into account.

By contrast, if CMS used the most recent income that HMRC could legally provide under the regulations, the fact that it does not reflect today's earnings does not necessarily mean the assessment is an administrative error—it may instead indicate that the statutory scheme relies on historical taxable income unless another mechanism allows the assessment to be updated.

A comprehensive guide

1. Identity and Personal Details

Check names, dates of birth, National Insurance numbers, addresses, and case references.

Evidence: official ID, HMRC records, correspondence, birth certificates.

Example wording:

"I believe there may be an error in the personal details used for my assessment. Please confirm the identifiers used to obtain my HMRC information and provide the audit trail showing how they were matched."

2. HMRC Income Data

Compare the income CMS used with HMRC tax records, P60s, Self Assessment returns, RTI data, payslips, or company accounts.

If income is irregular, explain why a historical average may not reflect current circumstances (where the rules allow this to be considered).

Example wording:

"The income figure used appears inconsistent with my HMRC records. Please provide the HMRC income figure, tax year used, and calculation showing how the weekly gross income was derived."

3. Shared Care

Gather calendars, court orders, parenting agreements, school records, travel records, messages, or other evidence of overnight care.

Example wording:

"The number of shared-care nights recorded does not reflect the actual arrangements. Please review the attached evidence and reconsider the shared-care calculation."

4. Missing or Misallocated Payments

Collect bank statements, payment references, standing order records, receipts, and CMS payment histories.

Example wording:

"Payments shown on my bank statements do not appear to have been credited to my account. Please investigate and reconcile the payment history."

5. Possible Duplicate or Historical Cases

Compare case reference numbers, opening and closing dates, and correspondence.

Example wording:

"Please confirm whether any historical CSA records have been linked to this case and provide an explanation of how those records have been treated."

6. Arrears

Request a full arrears breakdown, including every charge, payment, adjustment, and date.

Example wording:

"Please provide a detailed statement explaining how the arrears balance has been calculated from the start of the case."

7. Case History and Audit Trail

Ask for the chronology of decisions, changes, and information received.

Example wording:

"Please provide the audit trail and case chronology showing when information was received, amended, and used in calculating my liability."

8. Administrative Errors

Identify discrepancies between letters, telephone conversations, and decisions.

Keep a timeline of events.

Example wording:

"The information in your correspondence is inconsistent. Please explain which version is correct and the basis for the decision."

9. Escalating Concerns

If concerns remain unresolved, consider requesting a mandatory reconsideration, using the complaints process, or appealing where appropriate.

Stick to evidence and specific discrepancies rather than making accusations unless there is clear proof.

A guide like this is strongest when it focuses on facts, documents, timelines, and specific requests, rather than assuming misconduct. That approach generally makes it easier for CMS or an appeal body to understand and investigate the issues.