Magic money?

Here's a brainteaser which has been around for some time, but there may be those of you who haven't come across it before -
Three people go to a restaurant for a meal.  At the end, they are presented with a bill for £30.  They decide to split the cost between them equally, so each gives the waiter £10.  He takes the money to the cashier, who realises that the bill has been incorrectly totalled, and the true figure is £25.  He therefore gives the waiter £5 to return to the diners.  The waiter realises he cannot divide £5 equally between the three of them, and anyway they don't know what the true figure is, so he simply tells them they have been overcharged, apologises and gives them each £1.  The other £2 he keeps for himself.  So now each diner has paid £9 and the waiter has £2 - a total of £29.  What happened to the other £1?  As ever, I'll give you a few days to think about it.