Finished the repairs to the Red Baron. Chiefly, fitting the right kind of front derailleur[1]. Also crimped a new spade connector onto the dangling cable for the lights, and replaced the bottle cage which had been converted into a super-lightweight model by the slide along the road.
Then, out of curiosity, and since the bike was mounted in the turbo trainer, I had a go at pedalling it. FuckingHellHowPainful. I suspect that once the road rash heals, I'll have to stick to saddles for a bit until the bruising subsides.
Also swapped the Schrader tubes in the Brompton for Presta ones, because the great thing about standards.
[1] Recumbentists and tourists take note: FD-R453 is the magic front mech that shifts road-sized chainrings with a MTB-shifter-compatible pull ratio.