Magic money(2)

So, did you see it?  There is no missing £1 - it's just the way the story is told that creates the confusion, and the £30 is a red herring.  The easiest way of seeing it is to do a simple cash balance - the diners between them paid £27 - and where did that £27 go?  Well £25 of it went to the restaurant to pay the bill, and the other £2 ended up in the waiter's pocket.  All the rest is smoke and mirrors.