Hello everybody, just thought I ought to close this thread down with an explanation of what the cause apparently was.

I purchased a BOB that was driven by the Windows LPT port.....only to then decide to use a new pc for this build and hence use Win10. Win10 doesn't support the LPT port so I purchased a USB to LPT converter. Unfortunately I believe it was this device that was  causing my problem with weird circles. It was especially designed for CNC use and included 'software' that 'optimised' arcs. I extracted it from my machine to test offline but for some unknown reason it then ceased to function - so I was not able to investigate further or discuss with the manufacturer what the cause might be. Apologies, I am therefore unable to pass further comment and do not want to identify the manufacturer (as that wouldn't be fair) without having discussed with them and thereby proved the cause of my problem.

Suffice to say, a rebuild of the control interface of the machine with a USB BOB this time, has it running and moving as expected. I cannot see the BOB having any kind of influence on the shapes the machine  was being directed to cut and so the USB to LPT interface has to be the cause.

I now have an accuracy/resolution problem but will document/ ask for assistance for that in a new thread!