VAT is clearly marked on the receipt so it wasn't that. If it was something simple like this, I think it would've been obvious (if not to us, at least the manager working there!).

The bill was totally itemised, the total just wasn't the sum of all the numbers above it. I really wish I'd taken a picture of it now (I didn't really expect them to take it off me, I really expected them to explain that I was being a doofus) :(