I can’t believe it, this actually works. Thanks! 
Weird thing is, after going through all the official MS troubleshooting docs, and any forum advice I could find, none of it mentioned clearing the Steam cache.
I’m guessing it’s actually the Steam logout, clear all cached user data, login action that resolved the problem, rather than cached download files.
Whilst I’m happy that I now have the game back, this isn’t great Asobo. Logins fail, cookies / tokens expire, user data can get messed up … your software needs to deal with this.
Anyway, I’ll update my ZenDesk ticket with the resolution.