Anytime you intend to buy something online go to Quidco & log-in to your account there. Do a search on who you're buying from e.g. Curry's, then go to Curry's via the link Quidco give you (& check if there's any valid vouchers to use too). Buy the item you want from Curry's. Quidco gave you a tracking cookie when you clicked their link to Curry's. Sooner or later your purchase is tracked /verified & paid to your Quidco account. Might take a while but most folk leave it till they've got say 50 quid in their account then get it paid to your bank account.
Seems pretty reputable, lots on here use it.
I'm sure there's a FAQ on Quidco.