Thanks.
My phone is personal, and is not accessible to anyone else, not even my wife. I checked Settings -> General -> VPN and Device Administration and there were no profiles to delete.
As I said in my original post I had closed all apps via the app switcher, which is something I do every few days. I had also rebooted and carried out a hard reset to ensure the phone was as clean as possible. It was sometime after this that I had noticed that my “Parking” folder had been renamed to “Sea”.
I hadn’t added any new apps to the folder. In fact the most recent thing I had done was delete an app which I discovered only covered parking in parts of Belgium and Netherlands - not something I’m planning on doing in the near future.
IOS 15.5 has just appeared, so I will install tonight.
As there is no ongoing problem visible - I’ve recovered my settings app and have renamed my folder back to “Parking” - there is probably no way of knowing whether anything has been fixed or not. If it happens again, I’ll know, but otherwise …..
So I’m concluding that it is a bug, but can’t offer Apple any diagnostics.
The only other possibilities would be Pegasus or GCHQ, but as I am retired and have never had any high profile connections, that is incredibly unlikely.