I had a chance to revisit this and after triggering the ctd for downloading the same area in London, with the same settings as before, I restarted MSFS and allowed it to start in “safe mode”.
Returning to the manual cache settings and clicking ‘resume’ on the download has indeed allowed it to progress without the initial ctd I was experiencing.
So it appears that some marketplace downloads can indeed interfere with this manual cache process and cause a ctd.
The download concluded successfully without a ctd 
