Thanks for your suggestions, guys.
After stripping the drawing down to one floor plan and one view, the problem still existed.
I found that, if I opened a new file, and copy-clipped the floor plan and view into that file, the problem went away. It would however be extremely laborious to do this with the whole of the original file!
Went back to the original and tried changing to 'As Is Output' - and this did the trick, thanks Jim, so I'm hoping our consultants can work with that.
Keith