Go to "Toys & Games" tab and then "2 for 15 pounds on Toys".
Add a £9.99 item and then add one of the following items:
Fireman Sam Friction Fire Engine. 349/5557
Waybuloo Giant Floor Puzzle. 349/9151
Fifi Cook 'n' Clean Playset. 349/7232
It should charge you £15 but instead comes up at £12.50, worth probably playing about with and taking a printout of your reservation instore