If it was off by 5%, there could be some weird reduced-rate VAT @ 5% (https://www.gov.uk/vat-rates) not being printed out separately but included in the total nonetheless.
If I remember correctly, most booze is at the full 20% rate. I can imagine a mistake like this happening at least.
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) :(
If I remember correctly, most booze is at the full 20% rate. I can imagine a mistake like this happening at least.
Of course, if it wasn't off by 5%...