OK - I don’t want to get anyone’s hopes up, but I seem to have made headway …
I rang Apple again today and spoke to two guys who seem to have unlocked the problem I’ve been having. Caveat: I upgraded to Windows 10 for free - plus I have the latest iTunes installed.
The Apple tech guy advised me to go to Edit then Preferences. A box popped up which showed that syncing with the cloud wasn’t ticked. This certainly wasn’t something I’d done - I can only assume that it somehow happened automatically. I ticked it again and suddenly my iCloud returned.
This seems to have fixed the issue I was having BUT [1] iTunes on my PC spent ages syncing with iCloud and in doing so randomly generated loads of duplicate tracks; [2] these then filled up my phone - it somehow changed the automatic downloads switch, which I know I set to ‘off’ … so I’m having to weed through the music on my phone, removing the downloaded tracks which should never have gone on there.
BUT (again!) I appear to be free of the problem … fingers crossed?!?!?