SOLVED - After a few hours scratching my head, trying different solutions to fix the problems that others had suggested - many of them on this forum, thank you for your help in that guys - I was contemplating deleting and reinstalling everything (Zendesk weren’t particularly helpful I have to say) but then I tried something in desperation and it worked. See below for the process I did.
I’m convinced that the mandatory update was corrupt somehow, so I looked for the files/folders that were updated by that update that was originally suggested by @ToniGsxr600 in the following folder path:
C:\Users\YourUser\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\Packages\Official\OneStore
The 4 folders updated at the time of downloading the update were:
fs-base-genericairports
.fscontent
fs-base-ai-traffic
fs-base-nav
I moved these 4 folders to my desktop - DO NOT DELETE THEM! in case this solution doesn’t work for you and you can move them back again if needed!!!
I then tried to load MSFS again, and voila! - the mandatory update was again available and downloaded (much quicker than yesterday too), which placed new versions of those folders where they were. My simulator loaded without any further problem and is working perfectly again. Those files must have been corrupted somehow and that’s the reason that my simulator had multiple CTD’s when loading.
If you’re still having a problem, try this solution as it worked for me and it will hopefully work for you too.
I have now deleted the folders that I moved to my desktop too - just for housekeeping purposes 
Thanks again for your help/encouragement in trying to get this sorted, I really appreciate it and have avoided the dreaded prospect of deleting and reinstalling.