Thanks for the reply Gill, I just tried that and it didnt change it, however I noticed the  same box had a 'smooth polygon' checkbox and by unchecking it the marks vanished  :)

 

 

Tools:- Win10, Dell XPS8900, ZBrush, Marvelous Designer 11, Hex 2, PSP8. PSP 2019 Ultimate, DAZ Studio, Affinity Photo, Affinity Designer, Filterforge 11, flowscape,  Classic UVMapper, and several headache tablets.