Expensive, but one heck of a lot of network thrown in. If you must have an iP4S(!) and need lots of service then this is pretty cheap.
iPhone 4S 16Gb costs about £500, so this deal gives you T-Mob's Full Monty for about £13/mo, which is a steal (for a non-retention deal).....
Click through Quidco for £150 cashback. iP4S currently on offer at T-Mob on their Full Monty package - Unlimited internet, Unlimited texts, 2000 minutes to any network Plus unlimited T-Mobile calls
Phone is £99 upfront plus 24 x £36 = £963
Less cashback of £150 = £813
Effective cost/mo = £33.87
Expires Sunday 11th March