Many thanks to everyone who has replied to my original post. The version of
Molscript which we had was fairly old, so the contact information for Per
Kraulis was out of date. I have the program now, and I am very impressed
with the results, when fed through raster3d and ImageMagick. (If you don't
have these programs, check them out - you can find a nearby site to get
them from with xarchie).