A complete con even with the "deal". The "Oxy" is merely dilute hydrogen peroxide. Just use a normal carpet cleaner shampoo and add some of this to it "Sainsbury's Carpet & Upholstery Cleaner with Active Oxygen 500ml" £2 a bottle.
Often sold at around the £15/£16 mark from Argos. Unless you've got loads of carpets and clean every couple of months, 3 bottles of this stuff will last you years and years. The solution works well and has a pleasant perfume. Worked really well on the stairs which I did twice just for good measure. If you don't need Scotchgard protection, there are cheaper alternatives from Bissell, Rug Doctor or Vax.