The Natural Philosopher wrote:

over temperature cutout?

Thanks for the thought. I did try again after a lengthy interval and the
charging light did come back on. But when the charger showed the battery as
completely full, the battery's own indicator didn't. And again I only got a
minute of so of use before the tool died again.

Currently, after another (cooling down?) period the battery is charging
again. It will be interesting to see what happens this time.