Hi all,

Thanks very much for all the information. Sorry for the original
cross-post; I was expecting very few replies.

In answer to some of your questions, the clock is a WWII German
"Dehomag" slave clock, originally designed to be driven by a master
clock. It has no second hand. The minute hand appears to be balanced,
but the hour hand not. The clock is about 14" across, so not huge, but
it's exceptionally heavy as nearly everything is made from 1 mm steel
plate. Here's a picture of the clock:

http://www.mythic-beasts.com/~cdt22/dehomag.jpg

I can't decide whether to repaint it black, as it was when manufactured,
or to leave it grey. It's for my bedroom, so if it were to stop during
power cuts I wouldn't mind. It sounds like the 50 Hz grid frequency will
be a plenty accurate time keeping source, and it's an interesting
solution to the problem. We live in a pretty rural area so I'll try to
design a circuit which will be fairly resistant to noise. I had already
planned to put a Schmitt trigger on the input from the step-down
transformer.

Once again, thanks for all the advice. It's much more than I got from
the National Grid - the phone number on their website doesn't even work!

Best wishes,

Chris