Well, bugger me (as they say).
Took a bit of playing and clicking reset - but Avtrician had it right.
I've been doing the dual screen for almost two years without any full time problem - but if I tried to change anything, it sometimes caused glitches.
The "cable swap" has never worked - so i didn't even try it this time.
However, I noticed that the video control panel always recognised, and numbered, the monitors in the order in which they were connected - so, I unplugged both, plugged as Left = 1, Right = 2, hit reset and waited.
TaDa!!
To be fair to Shunter - that was exactly what I have been doing for the two years - drag/swap/drop.....but this graphics controler refuses to let me move things. Just not an option. All I "could" do was click to decide which would be the primary screen.
Sorted now, bunnies, thanks.