Ok small update for you...
I was messing around and thought to change the "Network Mode" under "Mobile Network Settings" ... ever since I got the phone it has been left on 'GSM/WCDMA auto' and worked fine until I updated to 2.2.. now I get nothing, I decided to see what would happen if I changed this to 'GSM only' which only supports 2g and it works fine coming up with the G (GPRS/2g) symbol, but as soon as I switch back to try and use HSDPA nothing! Its like I`m being stopped from receiving data downstream but the upstream is trying its best!
Theres only 3 things I can possibly think of that is 'could' be..
1. I did receive a new SIM card from O2 before the 2.2 update so that may have something to do with it.
2. The HSDPA Network in my area is down or playing up.
3. The Froyo update has caused it (it only started after this)
I guess my next step will be to contact O2 and HTC support to see what non response I get...
Will keep you updated incase anyone else is having same problem
---UPDATE---
I decided to try something else, I used to have an iphone before the HTC Desire, I took out my SIM and popped it back into the iphone and used the same Data Connection settings as on the desire, EDGE worked as GPRS did on the Desire, but I get the same problem on the iphone as I do on the Desire with 3G/HSDPA! ... This has narrowed it down in my opinion, it must be a HSDPA Network issue somewhere or something at O2s end of the equation, the new SIM card thing is sitting in the back of my mind now! On to O2! ... HASSLE!