I have found Mail.msi on my PC. It was in a different folder inside that .cache folder. The problem I have now is I get this error: -
If I use regedit to try and view that key, I get this: -
I'm logged in as an admin and the permissions appear to be set correctly.
I wish I'd just stayed with the older version of the program. Unless I can fix this, the only option now is to format the drive and reinstall Windows, which is a complete pain.