This was a fresh drawing. I wondered if it was something to do with the template I use, but I have created a new model exported it to a blank (i.e. no template) sheet, and get the same results. very strange...
Thanks Ralf - you've pointed me in the right direction, However there appears to be something weird (possibly unique to my machine). My settings all agree with those that you point to (apart from the scale being 1.00), however, if I change the scale, save the file and then reopen the drawing, the drawing is updated to now show the shading - a bit weird, but at least I have a workaround...