OK so potentially I have cracked mine as the download has been going strong.
1. Take everything that you have go so far in your official folder out. What I mean is make a copy to your desktop.
2. Go to the app settings of msfs2020 and uninstall.
3. Reinstall via the store.
4. Start msfs via the store. Once you have started to the download pause and close the sim.
5. Add the copied official folder back in overwriting anything already there.
6. Restart the sim and then continue download.
For what ever reason I think this might have solved my issue