Hi,
Here is an update - I really hate it when people ask for help but never let people following the thread know what happened in the end... (this isn't quite the end yet")
I did get the i7 machine to boot in the end and it does work fine but the memory will only run up to 5000 (even though that memory happily runs at 5600 with the i9), so Chup, you definitely have something about the compatibility and I must have got lucky with the i9 system!
Here is what happened:
I went to do further work on the i9 and realised that the BIOS flash had not worked, it was still on F3 (first release). Because I then realised that my flashing technique had not been working - even though the amber LED appeared to indicate that it had, I did the obvious thing and attempted to flash the i7 with F3 (the only version I had not tried as it was the oldest), and used a new pen drive to try it.
This got the i7 system working, and like the i9, at first it did not have XMP enabled which is normal, so I enabled it and it wouldn't boot though this time after a minute or 2 of thinking about it, it did what you would expect and booted into a mode with safe settings - much better behaviour.
I found that I could manually tweak the memory up to 5000 but no further. I then flashed F7A because I wanted to put the latest version of the BIOS on the i9 machine and thought best to test on the troublesome machine first (the i9 is for a friend). This worked fine so i9 also flashed now running F7A at 5600 and running well!
Meanwhile, Gigabyte have asked me to try a version F7C that they linked me. Don't ask me for a copy as its obviously not released yet and didn't seem any better anyway, same problems, HOWEVER - I re-read their message tonight and haven't followed their instructions regarding clearing the CMOS and some other things exactly so I will retry that tomorrow before I write off F7C as not a solution! I think its unlikely that it will make a difference but I can live with the memory at 5000 for now and hopefully eventually a BIOS update will make it properly compatible.
I am very busy with other things now as PC building is only a tiny part of my job, but I will keep you all updated until this concludes.
Cheers.