Just wanted to say thanks to everyone who recommended Corel Draw and say one thing - this program is in many ways easier to use than Micrografx Windows Draw or Serif, especially for the relatively simple sort of thing I'm doing! For example, subtracting one shape from another is considerably easier and gives better results in Corel, curve editing is MUCH easier. I suspect that it does get a lot hairier if you get deeper into it, and especially into the more artistic side of things, but it looks like the steep part of the learning curve is somewhat above the level I'm working at.
In short, I'm now pretty confident that I know what I'm doing, at least for now, and I'm getting on pretty well. I think that one of the things I may put into the Forgotten Futures Compendium is a deck plan of a Buck Rogers style spaceship, complete with silly antennae etc., as a sampler of things to come. Here's a screen shot of what I've done so far - as usual any mottling etc. is down to jpeg compression and won't be in the final files.
Anyway, many thanks to everyone who commented for persuading me to take the plunge. It's definitely worth it.