Surely this has been on the boards so many times over the years that the answer's well known?

There's no £1 missing. It's the way it's all been written that's confused people.

The proper charge was £25. So the proper charge for each was £8.33. The bar back had £2 which is 67p each. So each unknowingly paid out £9.

They were given £3 back so each got £1 back. That accounts for the £10 each they'd paid originally. QED.