Decided to do something about slipping 4th. on the hack/shopping bike. Cleaned a handful of clag off the transmission to find that chain, ring and block were a little worn. Looked at 4th. sprocket to find that it had become the five-and-12-half-tooth model.
New block, chain, rear mech and v. good 2nd.-hand ring (I hate putting new stuff on this bike, especially as the whole lot cost around 25 quid!).
Haven't tried it on the road yet, but seems OK.
Oh, also did pedals and wheels, so the bike should have a bit less 'drag'.