Hello,
It is maddening when instruments on the dashboard don't work and had exactly your experience with the clock in my '72 S2 Coupe.
I sent mine off to JDO Instruments. They are in Keighley, West Yorkshire. They replaced the old movement with a Quartz one and I have had no problems with it since. All guaranteed of course and no VAT. The only visual difference is that the new movement does not have a second hand.
I hope this helps.
Kind regards,
Al.