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HMRC internal manual

Self Assessment Manual

Records: maintain taxpayer record: SA mixed records

What is a mixed record?

A mixed record is where the records of two customers get confused so that data for one customer is entered on the records for another. So, an SA record set up for, or belonging to, one customer, shows the NINO (and possibly designatory details) belonging to another. However, there are data security aspects to the incorrect mixing of individual customers records which are of paramount importance.

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Common causes

1. An SA record for a customer created with a Temporary Reference Number (TRN) is matched by the computer to another customers NINO on TBS and they are merged together. This process is called ‘Automerge’.
2. Two customers use the same National Insurance number (NINO).
3. Keying error when the record is created, or updated, by capturing letters and/or numbers in the NINO in the wrong order.


The implications of the mixed record can vary depending on what systems have an interest in the individual and can include:

  • Corrupt designatory data across all linked systems
  • System output issued to the wrong person, such as a repayment issued to the wrong person, an SA underpayment in the wrong tax code, SA statements issued to the wrong person
  • Customers are open to identity fraud
  • National insurance contributions posted to the wrong account. This may not become apparent until a benefit claim is made or state pension becomes due
  • HMRC (DMB) trying to recover debts from the wrong person
  • Tax Credits or Child Benefit paid to the wrong person

Warning signs of a possible SA mixed record

  • The private address on TBS (linking through to SA and NPS) keeps switching between two specific addresses
  • Compliance cases to be taken up for enquiry where it is thought the SA customer is not declaring PAYE income as shown on a P14. It could be that the P14 does not belong to the SA customer, even though the NINO on the P14 and SA record are the same
  • The SA customer telephones quoting their UTR but fails security as the NINO and address on the SA record do not match the information that the caller is providing
  • The PAYE customer contacts us to say they are not an SA customer
  • The PAYE customer contacts us to say they have received a Notice of Coding for an employment that is not theirs and there is a UTR showing in Indicators (for example SA involvement)

For Example

David Smith is an SA customer. He is a self-employed taxi driver
Darren Smith is not in SA. He works at Tesco, and is NPS only

David’s SA record has Darren’s NINO and designatory details on it
David’s TBS record shows his NINO but does not show his UTR
David’s NPS record does not show his UTR

Darren’s TBS record shows David’s UTR
Darren’s NPS record shows David UTR

Note: On SA, the UTR is constant and does not change but any NINO, and other designatory details, can vary. On TBS, the NINO is constant and does not change but any UTR, and other designatory details can vary.


Establish who the SA record belongs to, or who it was set up for, and verify who owns the NINO’s involved.

No two mixed records will have become mixed for the same reasons or be able to be resolved in exactly the same way. Each mixed records case must therefore be reviewed individually, taking full account of all circumstances of the record(s) including establishing who the SA record belongs to and who owns the NINO’s involved.

Mixed record cases which cannot be resolved within local offices are sent to a central team. This central team only deals with those genuine mixed record cases and they have their own detailed guidance to follow. Any referrals to this central team after having followed the Action Guide at SAM101106, should be made on the proper stencil within the Action Guide.