Yeah I ballsed that. Let me try again.
So they pay £30, bill is £25, each of them get £1 each back, equalling £28, £28 divided by three = £9.33 recurring, which only comes to £27.99 as you can't get a 0.3 recurring of a penny (obviously), then the waiter keeps the other £2 = £29.99.
So...