Hmm... Before we get too excited, I have a nasty feeling I may have sacrificed my box to the cause I now can't install a firmware update from usb. The "hold down the powerbutton on startup" no longer works.
Hurrah - I've managed to flash my HD box back to normal . Turns out I'm completely stupid, and didn't try one important thing - About the time it broke, I fitted a usb extender lead to the usb port to save me reaching round the back all the time and save ware and tear on the socket - just a male to female job, no hub or anything fancy. Just thought to try it without that and it worked! Strange as the flash disk is quite readable with the lead once its booted up, but the light on the flash comes on later in the boot sequence if the lead is plugged in. I can only guess that it takes a little time to ramp up the power on the usb, and longer lead causes enough resistance to slow it down. One to watch out for in future!
Now to turn my confused "HDR/HD" box back into a "HD/HDR" - though I'd just got it all working nicely, its good that my box is no longer so "unique" and I can post scripts without having to get someone else to test them first!