Today I finally fitted my new pedals!
I used the crank I got from Panoramix, which doesn't match the other one visually but is the same length and does the job.
I also started dismantling the old pedal, still stuck on the crank. I'd never paid much attention to its construction before - an alloy cage is bolted to a body which is plastic, but tough and rigid, and the whole lot revolves on a ferro-cheese axle. Despite the materials I can't really complain about these pedals (VP brand, if you want to know); they've been on the bike since it was new almost 13 years ago and they're still in good condition, the bearings are smooth, etc.
Trouble is I'm now not sure it was a good idea to change them! After many years using toe clips the idea was I'd switch to SPD for "rides" but with flats on the other side for riding round town. I haven't fitted the cleats yet but a short test run this afternoon showed me I really don't like riding on flat, unclipped pedals now. I actually had similar pedals some time ago on a different bike but don't remember it being a problem - I don't think I ever used them in 'normal' shoes though. Oh well, I guess I'll give it some more time - I don't think I'll be clomping round shops and so on in cleats. Well, maybe, time will tell!