Dug his heels in. I noticed he had stopped at 8.45. Tried to reboot; just sat there thumbing his nose at me. Plugged into computer. Time corrected itself but no sign of recharging battery. Brought up Gamin Express and tried to synch via that. Dug his heels in further and refused to connect. No screens on him accessible. Google search brought up the advice to hold top left button for 20 seconds and repeat till watch switched off. Did that, rebooted, all is well again. How strange!
Just posting this in case anyone else has the same problem..