Thanks for explaining the version checking requirements of the installer. So it can't be that, I agree.
I downloaded the installer afresh and put it in a lower level folder where all my other installers are (on a separate fixed hard drive). I disabled the AV. The installer still crashed when I ran it (as admin).
The only thing I hadn't tried was running it from a folder on the same drive as P3D. This time it ran and installed as normal. I've never seen this type of thing before, as I've been routinely running installers from my backup disk with no problems at all.
I'm happy to admit I was wrong and you were right, but I'm at a loss to understand why, with nothing obvious having changed, I can now only install FSUIPC from the same disk P3D is on, and not from the desktop at that. Yet I was able to reinstall my PMDG 747 and 777 addons today, running them from the desktop with no issues.
Thanks for your help
Kevin