Quick drive to shops to test that it's still working, and the battery light, charging fault lights back on again. My unlikely water cure seems not to be a cure at all.
Multimeter saying just 10.14 across the terminals now which I guess is just what the battery has dropped to.
Is it possible that the alternator is refusing to supply a charge when the battery has dropped below a certain level? It had charged up as high as 12.5 before I put it back in the car this afternoon.