Do not assume that if a refund has been issued to you that that means the Revenue have agreed your claim. At that stage no human will have been involved. They have 12 months from the submission date to enquire into your return under section 9A, longer where they make a ‘discovery’ or where fraud is involved.
They have an explicit right to enquire into the completeness and accuracy of the information provided – and their enquiries could cover anything from straightforward requests for information on individual items through to full reviews of books and records.
HMRC do not have to justify why they are enquiring. It could just be on a random selection basis or it could be selected by reference to either information in the tax return itself or other information in their possession.