A nice anonymous person just pointed out very politely that I'd completely got my terms muddled. I've been happily banging on about "fixies" as if I knew what they were, but of course it turns out that I don't. I kind of thought that anything on wheels with just one gear was a fixie - but in fact a fixie is one where you can't freewheel and (I now think) has the brake as part of the pedal system ... (correct me if I'm wrong!).
The moral of the story is don't look for clever answers here - it's just a place to air issues to do with biking. I see myself as an enthusiastic amateur who picks up little bits of knowledge over the years. The real experts are out there! I suspect I need you more than you need me!
Love and karma all around. As they may have said in the 70's.