For if you didn't get the great £25/month deal http://www.hotukdeals.com/deals/htc-desire-hd-25pm-t-mob-e2save/781556
This deal is probably not for everyone, only those who can remember to claim. It is based on 6 months free line rental. Luckily mobiles.co.uk cashback seems quite simple as it is web based (https://securesite.mobiles.co.uk/submit-bill.html)
This is a 24 month contract at £30 a month. It can have 6 months free if you redeem at months 6, 9, 12, 15, and 18. It is the Web 900 24m tariff (scroll down). This is slightly better than the £25.00 a month in that is has 300 more minutes.
Some people claim they are able to switch their internet to the free HSDPA 3gb for Android on Tmobile, so you can switch your booster (to say unlimited texts). This may not happen though so be wary.
If you are punctual (use your phone's built in calendar), you can submit for cashback for 6 months free line rental, making your phone the equivalent of £22.50/month.
For comparison, the 600 degree deal is 25/month, total £600.
Topcashback available for £50. Making the phone potentially £490, or £20.42 a month.
This all depends on your luck and will likely fail.