Turbo trainer. Ebay bargain from a local seller (who has very kindly offered to drop it off on Monday, which saves me trying to transport it by bike).
I expect I'll have to do something clever to make it work with a 20" wheel, but how hard can that be?
Well, I've now got the trainer, and while it doesn't look too tricky to molish an appropriate extension bracket, I've just discovered a more serious flaw in the plan: the bottom of the ICE luggage rack sits almost exactly in line with the rear axle. Grr.
We've just had a serious head-scratching session, which involved just a little bit of swearing and the experimental undoing of various bolts.
As I see it, the options seem to be:
a) take the sides of the rack off
b) rollers
Neither of these are particularly attractive, as a) involves 6 bolts and the use of a third hand, so isn't something we'd be doing with any regularity, so is only reasonable if combined with the purchase of a rack bag and b) is, as far as I can see, liable to result in the chain fouling the roller without further fettling to accept a small wheel (and would be useless with the other bikes).
Fneh.