In reply to gethin_allen:
I use topcashback and quidco; topcashback generally gives a little more cashback if you have the choice.
It almost always comes through, but it isn't absolutely guaranteed. Make sure to use it with advert blocking and the like disabled, and follow the links from it to the retailer on the way to making your purchase/signup.
As long as you take the cashback as an extra bonus, rather than buying from someone just
because of the cashback, there really isn't anything to lose.