I'm very wary of this sort of thing. This, in their terms and conditions, worries me:
9. You are not entitled to a Cashback Reward in respect of Approved Transactions for which We do not receive the whole of the commission due from the Participating Retailer.
22. You shall contact Us only and shall not directly contact Participating Retailers or Partner Services in connection with any aspect of the Rewards Program, whether for the purposes of raising a query, seeking further information or making a complaint/claim.