Well I've done some more investigating and have some suspicions about the nature of the problem.
My thoughts are that dried-up gear lubricants, combined with dirt/debris/spiders etc. are probably a significant factor and affecting both original and replacement units.
Has anyone ever taken these things apart and have any idea what sort of lubricant (oil, grease or what) is used and what the gears/stops are made of (metal or nylon probably)?
Further is it possible to take them apart for cleaning and reassembly or at least flush through with lubricant and squirt in something suitable that will last?
I can't see any obvious way in for either purpose, but that doesn't mean it's impossible.
I have had a communication back from A8parts and it wouldn't be appropriate to comment on that other than to say it was fair and reasonable. 
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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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