Top marks to Brixton Cycles, who sorted me out, trying to get the parts together to repair my mech. I had to stop the assistant from opening a set of jockey wheels, and decided to buy a Deore mech (partly inspired by the way in which the chain had been skipping across to the larger sprocket and binding - not so much fun on the A3
).
So I have a shiny new derailleur, a glistening KMC chain, and a new bolt for my cleat (did I mention that had worked itself loose this morning, too). And because the delightful Butterfly came by with a tandem rear wheel, we have a shiny new freewheel.