Thanks very much for all your help, I’ve sorted it.
That guide was very thorough but after going through it and the associated Windows/XBox Game thingamajig stuff, it still wasn’t working. However, it made me realise that this is probably a user/login/credentials type problem…
What solved it in the end was a simple, log out of my xbox user (which kicks you back to the beginning of MFS) and log in again. On doing this, hey presto! everything reappeared (I’d also lost a few other aircraft that I hadn’t noticed earlier).
So thanks again for all your help and advice, I really appreciate it.