Thanks Umberto for the guidance. Very helpful.
Problem was resolved although it was a little more complicated than originally thought.
Turns out that when I ran the suggested Universal CRT, which is part of the Visual C++ Redistributable for Visual Studio 2015, to uninstall and re-install this (the 64bit version for Win 7 64bit), the install also failed. The associated log file provided an error which I traced back to Windows 7 SP1 not being installed. Looks like VS2015 has a requirement to have this installed.
So I attempted to install Win 7 SP1 but this generated yet another failure and aborted the install. This time related to the automount policy (see here for more details on this solution...
https://blogs.technet.microsoft.com/joscon/2011/02/17/windows-72008-r2-service-pack-1-fails-with-0x800f0a12/) .
With the Automount enabled I was able to install Win 7 SP1. Once completed, I re-installed the Universal CRT (x64 bit version) which was successful and then from that, re-installed GSX using the full installer.
On FSX Accel starting no Couatl.exe error was shown. FSX SE with GSX also working.
I hope this is useful for any one else that comes across this issue.
Regards
Mark