Probably a long shot but having gone to do what I assumed to be the usual repair to the driver headrest, I discovered part of the gearbox is missing - the bit where the cable connects in which I think also has a cover on it; the main drive gears are still in place. Consequently the cable is spinning happily but connected to nothing. No idea where or when this went as I've not had the back off the seat before, presumably some PO bodge. Realoem says its 52101927950 but that looks to be a bit more than I actually need and whilst still available, is nearly £100...
...incidentally the passenger side *did* have the more usual issue, easily fixed by trimming the cable sleeve
Electric headrest gearbox
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