"it's so easy to redeem your £15 offer...
1. Join Quidco, for free
2. Make a purchase with Zavvi of any Lego product
3. Await your £15 bonus
We'll credit your Quidco account with your £15 bonus by the 18th December 2016."
Great for someone who is looking to buy Lego anyway, in case they don't pay out I guess. I have always been paid cash back, as promised, from Quidco.