Ok all, it's working now. Unfortunately I cannot tell you for definite what the issue was as I have done several things all at once!
First off, it is a completely NEW PC with a 1TB SSD (plus 2Tb HD for good measure!) and completely fresh Win10 install, new Mobo, processor, graphics card, even a nice new case! The works. The only things the same are the 2 x 24inch monitors, 1 x DVI and 1 x HDMI. They are now running off DP to appropriate converters. Hopefully a 27inch freesync DP one to come soon, budget permitting!
I built the PC using the HDMI monitor (with DP to HDMI converter) and now I have connected the other monitor the HDMI one insists on being the 'primary' where Steam runs all games.
So, I had also uninstalled Elite due to problems as above prior to connecting 2nd monitor. I then remembered I had copied the folder of log files across to try to make my keyboard and joystick setup a lot easier so I moved these elsewhere … I am guessing these log files MAY contain some config details for graphics??
Then I upgraded the Graphics card software to the latest 20.7.2 version and went and reinstalled Elite. So, it could have been the graphics drivers, or it could have been the old log file … or even a combination of both? Anyway, it all seems to be working fine now. Thanks everyone for your help and suggestions! Now, how do I get it to run