Submission is not approval
HMRC aims to process 85% of claims within 40 days. In 2024/25 it processed 90% within that window. Processing is not approval of a claim, and HMRC can still open an enquiry. Usually, it must do this within 12 months of the filing deadline or, if the return was filed late, 12 months from the following quarter day. In some cases it can go back further if it believes something was wrong.