Thanks Adam. I did think of an AC reset, but there isn't anywhere obvious.
Anyway, she's up and running again. The lithium cell was low, straight out of the packet
I bought 5 x Maxell CR1620s and the first one I tore out from it's packaging (and used) was only measuring 3.0V. I'd forgotten that they should be about 3.3V when new.
The old one was measuring 2.8V, and neither this, nor the new 3.0V one would power up the watch.
It seems the Braun is smart enough to keep the watch running on a low voltage (while showing the 'Low Battery' warning), but won't power up from such a source.
Thanks for your help gentlemen
Rgds.