@pendeanand @maxim_k thank you both for your input, it seems @pendean was on the right track !

The monochrome ctb file did not help matters, however, after I tried many different more settings, printers, laptops etc etc to no avail, in the end I un-installed ACAD and installed a fresh version but did NOT migrate any settings, plot files, linetypes etc

When opening the file again I used the standard monochrome ctb and bingo all fine, so it seems that the migration potentially corrupts the plot files !!!

I then duplicated that ctb and put back my colours etc and it all works fine now.

So .............to anyone with similar issues  - on first start-up of ACAD after reinstalling; when you are asked if you want to migrate your settings, don't transfer the ctb files. When I did mine I simply refused all settings, it should be possible to just uncheck the plot files; but as I don't have many custom settings I didn't think it worth it

Also a point to note - if you have this problem and try to resolve it by saving back to 2017 it will transfer the bug !

Many thanks again for all your help, I can get back to work now