This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

Enquiry Manual

Working the Enquiry: Claims of Non Taxable Sources: Supporting Evidence from Third Parties

If the taxpayer supports their explanation by statements from another person, you will need to consider whether those written statements should be regarded as sufficient. You might write to the other person, meet them, or suggest their attendance as a witness at any appeal hearing.

Such a person should be asked to relate, in detail, the circumstances in which they found out what you have been told and should be tied down to specific facts within their own knowledge. Generally speaking, if they cannot specify amounts, their statement will be of limited value.

A sworn declaration should be accepted only if the witness is not available for interview and examination. In this case also, unless specific figures can be sworn to, the statement will be of little use.