There was a bit in the paper about Click Mobile, which offered cashback deals as a "third party" retailer. Third party retailers are not regulated by Ofcom, nor can the industry's ombudsman investiage complaints about them. So if they close up shop - as Click Mobile has - you're left hanging.
I would be very wary of any cashback deals if they are not DIRECTLY through the actual provider, i.e. Orange, O2, Virgin, etc.