Entirely my fault but just discovered I've been calculating my companies flat rate vat wrong for the past 2.5 years - all the time I've been vat registered. I've therefore underpaid HMRC to the tune of £6.5k. Fortunately it doesn't look like I'll be fined or have to pay interest as the sum is less than £10k, but I'm annoyed all the same. Grrr!!!!