So reporting back on those tests and steps:
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Optimised (trim needed doing on C, other disks and partitions were OK)
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Chkdsk = no problems.
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SFC scan now = Windows Resource Protection did not find any integrity violations. (note that I have had to use this tool in the recent past when the Batch file that swaps the on-screen keyboard to the newer style on this device, somehow started causing problems - after 8 months of being totally fine - with MSFS (not loading, crashing on splash screen) and the SFC scan fixed violations. I have now left it with the default KB and not had that problem again, and running the tool now has no faults)
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Runtimes. I had less than you = I have run that batch tool now but still 1 missing = C++ 2008 x64 9.0.30729.17
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Windows update after reboot had this = 2022-05 Cumulative Update Preview for .NET Framework 3.5 and 4.8 for Windows 10 Version 21H2 for x64 (KB5013887)
5b. I also installed the “Optional quality update”: 2022-05 Cumulative Update Preview for Windows 10 Version 21H2 for x64-based Systems (KB5014023)
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Did memory scan using Windows Memory Diagnostics. No problems detected.
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Not sure how to use that ProcDump. I need to download the EXE from Git? When I double click the file inside (e.g. procdump64) and run it, it just does something quick in a popup and I can’t type in a command anywhere.
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Malware may be another possibility. Or corrupt regisrtry. Not sure what are the best tools to use for that these days?
Having a BBQ now so will try and do a test flight with empty community (except the stuff that JustFlight install directly - that’s hard to remove temporarily) later tonight! Thanks again 
EDIT: Figured out what I need to do for ProcDump (put on C drive, go there with cmd cd prompt and then use your code, but I get this:
C:\ProcDump>procdump -ma -e FlightSimulator.exe
ProcDump v10.11 - Sysinternals process dump utility
Copyright (C) 2009-2021 Mark Russinovich and Andrew Richards
Sysinternals - www.sysinternals.com
No process matching the specified name can be found.
Try elevating the command prompt or using PsExec to make one as SYSTEM.
psexec.exe -s -d -i cmd.exe
procdump.exe -accepteula …
C:\ProcDump>procdump64.exe -ma -e FlightSimulator.exe
ProcDump v10.11 - Sysinternals process dump utility
Copyright (C) 2009-2021 Mark Russinovich and Andrew Richards
Sysinternals - www.sysinternals.com
No process matching the specified name can be found.
Try elevating the command prompt or using PsExec to make one as SYSTEM.
psexec.exe -s -d -i cmd.exe
procdump.exe -accepteula …
C:\ProcDump>