No, Mr Doe - I should be thanking you!
The fix above worked - after I made sure to delete the two useless "rules" I'd dreamed up earlier!
The back-up took an age, due to Bitdefender throwing in a spanner or two with this semi-persistent irritation:-
"An unauthorized app attempted to access your backups.
App: /System/Library/CoreServices/backupd.bundle/Contents/Resources/backupd
We blocked the app to prevent it from altering the content of your backups."
That nightmare has now gone & the back-up is done. As if I'd ever want to revert to the same problems as this morning!
I'm truly beginning to wish I'd never "up-graded", as none of this has happened before!
Part of me is thinking that I should now stick with the new, faffy wierd way of deleting, by using the "move to" method, and eventually, when hordes of others users have the same problem - there will suddenly be a magical fix, from Apple.
But the absence of thousands of others experiencing the same thing, had me thinking it must be down to some very specific configuration, or even a minor, but hard to find, change of settings!
I've not abandoned this, as I know it'll niggle me enough to keep me prodding, poking around & changing things.
And if I find anything anything that seems even remotely useful, or better still a full fix - I'll be straight back here, getting it down, before I get a chance to forget how it was done!