Whew, the problem is fixed. The magnet is supposed to be 3mm from the sensor according to the paperwork, though the LBS told me 4mm... so I experimented and moved the magnet just a little closer to the appropriate end of the sensor. It worked - I held the bike up and moved the crank a couple of times and saw both an average speed AND the odometer changed slightly. Success!

The magnet wasn't all that hard to move, perhaps when I turned Stella upside down the other day the magnet shifted a little? I don't know how likely that is, but it now seems to be working with the right side of the bike up

And I won't be turning it upside down again - though it was always on carpet with a blanket on top of the carpet - no scratches and with proof the computer works I know it hasn't been damaged either.

Thanks for the tips!