Interesting, but I just discovered something:
PO stated in his ad that the clock didn't work. I saw one on eBay at a reasonable price and bought it. Had it tested by a Porsche mechanic and he stated the eBay find worked just fine.
I finally got around to pulling out the defective one and inserting the "new" one, only to find that it didn't work in the car either!
So I deduced that I probably have two good clocks and some kind of wiring fault in the dash. I should have checked that first and, if you have a "bad" clock, it would be worth dragging out the volt/ohm meter
before sending it off to anyone!!
Tom