The Missing Pound

Three people enjoy a meal at a Thai restaurant.  The waiter brings the bill for £30 so each person pays £10.  Later the chef realised that the bill should have only been £25 so he sends the waiter back to the table with £5.  The waiter was not very good at Maths and could not figure out how to divide £5 so he gave each person a £1 coin and kept £2 for himself. 

So… the three people have paid £9 each for the meal since

3 x £9 = £27

The waiter kept £2. 

£27 + £2 = £29

What happened to the missing pound?