Thanks to all who replied.
It turns out there are approximately 55 CAD programs available at the moment. Easy enough to weed out those I can't afford, but a nightmare trying to select from the rest. After looking at a model engineering forum I am probably going to try QCAD Professional, partly because it will run on Linux, and partly because for something like this I think I'm safer with a paid-for product which is perhaps less likely to vanish into the ether one day. Well, I can hope.
Interesting to read about Phillips Digital Print - they are the preferred film maker for G&H so I will be using them. I like the idea of seeing the films before sending them to the etcher, for a beginner that sounds like a good way to go. I'm expecting a few failures before I have my first etch in my hands.
Michael