This worked for me.
I done another clean install this morning.
I download both the 'Full GSX Installer' and the
updated 'Standalone Addon Manager'
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Installed the 'Full GSX Installer' first and then the 'Standalone Addon Manager'
Started FSX and GSX. All working fine.
Just to test in case it was a fluke, I shut down and rebooted the PC. This time I also started my Active Sky Next software and then FSX. Loaded a saved flight and tested GSX, and so far it all works fine. (fingers crossed)
Hope this works for others.
(The new fuel truck is great) Nice piece of work.
Not sue if I should take a chance and install the latest FSUIPC version though. That might just be asking for trouble.