just thought I would share my experience with changing the button circuit board in the Desire.
My "back" button was needing a harder and harder press, so it was time to seek a replacement.
I managed to find a UK supplier on Ebay luckily (its referred to as HTC DESIRE G7 OPTICAL TRACKBALL & KEYPAD MEMBRANE FLEX).
There's a video on YouTube - that will show you exactly how to dismantle the Desire.
Removing the old part and replacing with the new was extremely easy, so I would encourage anyone who's Desire is out of warranty to give it a go should they come across a similar problem.
regards to all