Small errors can be corrected on your next return; larger ones must be disclosed to HMRC separately. The threshold turns on the net value of the error and your turnover — below the limit, adjust the next return and keep a record of what you changed and why. Above it, or where the error was deliberate, it needs a formal disclosure. Deliberate errors must always be disclosed regardless of size. Penalties depend far more on behaviour than on amount: a careless error disclosed unprompted attracts a much lower penalty, often nil, than the same error found by HMRC first. So the instinct to quietly bury a mistake is exactly backwards. Interest runs on underpaid VAT from the original due date either way.