A splendid day to be on the bike. Well, I was off it at this point, as one of my riding friends dealt with a puncture. Apologies for the badly mismatched wheels, but as I set out, I thought that the back wheel was rubbing: on inspection, I discovered that the wheel rim was cracked around three of the spokes. It wasn't a Shimano wheel, but one that ought to have lasted longer than it did... you can try to guess what it was, if you like.
Anyway, fortunately I had a spare new back wheel lying around, and it took me just 15 minutes to remove the tyre, inner tube and cassette from the old wheel, attach them all to the new wheel, and get on the road to meet my friends for the start of the ride. Crisis averted.