Another option for this:
£200 cashback for ALL 12 month SIM Only £25+ tariffs From £8.33 after Quidco cashback for 1200 mins / unlimited texts!
http://www.quidco.com/o2
So £8.33 * 12 = £100
+ HTC Desire sim Free £390
= £490 + only 12 months.
thats what I have just done.
Also phone is unbranded and unlocked = resell higher.
That's equivalent to £40/month though