Getting HMRC to refund overpayments
Here are the rules in English:
If the overpayment is less than £10,000 you can make an adjustment on your next return.
If it's more you have to make a specific claim setting out how the error occurred, how much was overpaid, and when.
Either way, there is a time limit of 4 years. More and you have lost it.
If it's output tax, HMRC can apply the condition that you refund the overpaid tax to your customer. Not to do so is what they describe as unjust enrichment, & if this applies they can refuse to pay.
HMRC can take some convincing that unjust enrichment doesn't apply.
We are aware of the circumstances where you should be able to attract a refund and can put forward compelling arguments to counter HMRC claims of unjust enrichment in many cases.