Not sure if this should be in the vouchers or deal or misc section, but anyway.
To do this:
Go to GAMESTATION and buy 2x £4.99 games (both different)
Then go to go GAME and do the trade any 2 games in + £5 and buy Proffesor Layton.
That's the £15 cash used.
Then you need to ask them how much trade in it is worth and ask for them to print out a reciept (It should be £20). You can trade it in here or if you want the extra £1, go to gamestation and they will beat that price by a £1..