Looks like you can pretty much get 20% off if you buy 2 Lego sets the same value.
Also some good other toys included.
Update: According to tests and purchases (mine included) it currently becomes a flat 20% discount off all items.
- Save 40% off the second item when you purchase 2 items across Toys, Lego, Costumes, Kids Art & Craft, Books and Paperchase.
*Purchased in one transaction. Not in conjunction with any other offer. Excludes: Clearance, Board & Card Games and selected LEGO ranges (Art, Creator Expert, Ideas, Super Mario, 71043 Harry Potter Hogwarts Castle, 76139 Batmobile and 42115 Lamborghini Sian FKP 37).