This forum is amazing!  So much knowledge and experience..

So, I will clean and check out the magnet and swap the gypsy from the secondary windlass too to see if that makes a difference. Also, I will check that the sensor has not slipped down. I can see it is about 3mm below the top surface, which seems ok according to Bill's table of distances. (It's a grey sensor)

The consensus seems to be that there is no connector in the cable-run, in which case the trouble is either with the sensor or magnet. 

If I need to replace the sensor, it seems I can either drill out the old one or maybe knock it out with a 10mm dowel and mallet?  The grey sensor kit comes with a magnet, so I will replace that too. Getting my hands on the sensor kit while cruising in Greece will be a little tricky.

Thank you Vladan, Dimitris, Nick, Roque and Bill. I have been helped up this little learning curve very quickly :-)
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Dean Gillies
SV Stella *****,  Amel 54-154