You'll need to buy the phone @ £379, plus top up the payg sim for £10 (which will only work for calls etc and is non refundable or transferrable) then unlock it for another £16 or so making £405.
The unlock works fine - I have done it and have used t-mobile in my HD7.
if the phone is bought online or over the phone do DO NOT need to top up the payg sim, saving you £10! you only have to pay £10 when you buy the handset on payg INSTORE,
hopes this helps a few!