John Pagett G4YTJ
Between you you've solved it! Bob was right about the 100k resistor. When I built the kit I put a wire link in instead of the 100k. After fitting R3 I was able to establish comms and connect with the target. I then had issues with verification failure but this was down to me trying to run the target via the USB power. That doesn't work, even with the LCD and Synth boards removed - Thanks Alan. You need to run the target off an external PSU (although the current displayed on a digital meter doesn't seem above 40mA).
As part of my fault finding I tried to roll back to V3.04 which used both hex and eep files. Interestingly the eep file programmed and verified with the U3 powered from the USB, but of course the hex would not verify.
The U3 has't seen any real use, because I haven't had any HF antennas up. That's now been rectified, so it may get an airing. Part of the reason for reprogramming was as a practice for when my QCX gets built, in case there are updates for that.
So now the display shows "Diagnostic mode" on power up. Time to get the manual out and sort it out!
Thanks again for the fast response gents. I really appreciate the ethos of this group with its willingness to share knowledge.