Double Taxation Applications and Claims: repayment interest: What is repayment interest
ICTA88/S826 provides for interest to be added to a repayment of income tax, or a payment of tax credit, to a company which is not made by the last day of the claimant’s accounting period in which the income was received. This interest is called repayment interest or interest on tax overpaid. For guidance about entitlement to repayment interest for companies not resident in the UK see INTM333520.
There are examples at INTM333570 and INTM333580 that show you how to work out the period for which repayment interest is due.