I found the solder fuse like in the later clocks, but this wasn't split. In fact, when I moved the wheels by hand so that the contacts on the flywheel met, putting the clock on a battery made them open again, so it had to be something else that was wrong (& likely to be non-repairable at home).
I then searched the 'net & came up with this CRF post from 2006 :
Just a tip for anyone who is repairing their series 1 clock:
The earth wire must not only be fitted to the terminal but must make contact with the outer can of the clock, as that provides the earth for the light. The contact has a special shape, with 'claws' that go under the insulator: