Talk about going round in circles! Finally fixed this afternoon. 
After some rummaging I decided it was probably the reed switch, so I ordered a replacement (you get the magnet too) on eBay - cheap as chips from China and it took less than a week to arrive.
But when I bench tested it I realised it was the wrong type, I'd got confused between NC and NO (Normally Closed or Open) not least because the terminology for alarm switches is sort of the reverse of what you might first assume. "Normally" refers to the situation when the control magnet is next to the switch and the commonest type (for doors mainly) is NC. But I needed an NO! Can't send it back to China so it will become a spare for my house alarm. 
I then remembered that my original installation used a 3-wire switch that could be set up as either NC or NO so I decided to order one of those - from China again. Again, very cheap.
Then today, while waiting for that to arrive, I decided to check on how much slack I had in the wiring and how easy it would be to change either component (switch or magnet). Plenty of slack so I took almost everything out and decided to check it all. Also confirmed the light on the dash was good. The magnet seemed to be OK as it triggered my new (but useless) switch. And the old switch, off the car, also seemed to be ok which puzzled me. 
So I went back to the wiring and eventually found a poorly-crimped connector in the extra wiring I added so long ago. Sorted that out and reassembled everything and it's now working fine. 
Should have checked it over first rather than assuming where the fault was, though with parts so cheap it sort of made sense to have them anyway. 
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions and extended leather. Ski hatch retrofit. Aftermarket reversing camera. DVB-T and XCarlink now removed as redundant.
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation".  (Even rarer than a D2 S8!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. Agate Grey/Platinum. Every option (I think) except electric rear seats, Tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon. Added Auto-dim mirror, Leather seats, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
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