I took the machine apart yesterday and the left hand extrusion was a bit rough along the rails. So I went over with a bit of wire wool. I also put the belt fastnersI found on thingiverse at https://www.thingiverse.com/thing:2564431
These helped me tighten the belts again with the same tension each side.
All is now working great and I am getting some really good engraving results.
Thanks for the help.