Thanks, John. I'm not finding it half as taxing learning Sketchup as I am GIMP or Blender. Vue also took me a long time to grasp properly, but I hit a wall with that since it doesn't allow any real modelling. Sketchup seem to understand that 'user friendly' goes a whole lot deeper than a standard list of menus, and their training vids are both excellent and totally free (which is sadly more than I can say for Blender these days).
Thank you so much for the style advice!
I have lovely muddy grey lines now. They aren't half as distracting when you zoom out to view the whole model
Curves - I successfully created both a dxf and a dwg file of contour lines in my 20 yr old Corel programme - lovely and smooth they were... but on importing them not only had they been converted to straight segments, they were in lots of tiny individual pieces! LOL! Maybe pro is different, but I think I will carry on drawing my jerky contours in Sketchup, since at least the segments are all in one long string and can be moved in one go.