It's pretty straightforward in my cae, Eliza. Overpayments of small amounts such as these are due to tax being deducted at source from savings interest payments and it is later determined, once the whole year's financial records can be assessed (by the Revenue, using the tax return), that no tax should have been deducted, resulting in an overpayment and a refund.

But in your case it could be a simple cock-up. (Oooo-er, missus, LOL).