www.quidco.com costs £5 pa (taken from your first cashback so nothing to lose, or you can use
www.topcashback.co.uk same idea.
Basically you register with them and use their links to retailers websites and order from there,
Halfords is a little different I think, in that through quidco you register your credit/debit card, then everytime you use in shop you automatically get cash back.
Works great, I have got about £900 in a couple of years, mobile contracts can pay £100+