So to clarify, you pay £20 per month then claim cashback in months 3,5,7,9,12. Cashback claims are submitted online and they are paid via BACS (I called CS to confirm this). If you are concerned about forgetting to claim, it's a good idea to set a reminder on your phone for each of the claim periods.
To make it an even sweeter deal put the purchase in your basket and then access the site via Topcashback who are paying £4.25 cash back on top (as always this is not guaranteed but mine has tracked ok)
So potentially this ends up as £7.15PM if the Topcashback pays out!