We have made an admin error when paying VAT on behalf of our client(Client-A). Mistakenly, we've used the Client-B's VAT number as a reference and also settled Client-B as normal. Understand the payment will be allocated against the incorrect VAT(correct as per what we've put as ref). HMRC website doesn't have anything to correct this situation. How do we go about correcting this silly but grave error on our part and is it ethical to keep client under all communication/developments. Keen to know what others have done to fix such error - appreciate your advice please!