I could understand if something was a few pence out and it seemed like it could be a rounding error; but exactly £1.50 on a bill of around £20 doesn't seem like

I could understand if something was a few pence out and it seemed like it could be a rounding error; but exactly £1.50 on a bill of around £20 doesn't seem like it could be rounding/tax/etc. (In the UK, all prices shown in the menus are inclusive of VAT and then the total on the bill shows how much of the total is the VAT, so both the amount you can add up yourself from the menu and the line items shown on the bill should match the total you're paying perfectly).