Anyway, tried a few ROMs (BeanStalk 4.4.255, MildWild 8.0JG, MIUI XJ 2.3.7, OxyZM 1.2) - all load but always the Desire runs in Safe Mode. This is shown by the words "Safe Mode" in the bottom left of the screen and a failure to run any downloaded Apps.
Other ROMs (BCM-RC4.1U2 and Carbon KK Unofficial 20140430) run without Safe Mode and only minor glitches (most prominant seems to be an inability to enter 'standby').
Anyway, I'm trying to figure this all out. I now know the Optical Trackpad does not 'track' (ie does not react to swipes). Took me a while to figure as I had no idea what it is supposed to do. It does react to pushing (as some type of 'OK' button).
So, having read loads on the web it seems a failure in the Optical Trackpad makes the Desire run in Safe Mode. Likewise its failure seemed to interfere with the volume down in the original O2 firmware. The failure does not seem to be a problem newer Android versions (I think the BCM and Carbon ROMs use newer versions).
Anyway, anyone got any experience, ideas or suggestions. The phone is a great toy and I love a challenge, been good fun getting this far on a "dead phone". Really impressed with the screen, given I had never used a Smartphone before.