Well, after much stuffing around I separated the headrest foam cushion into 2 parts revealing the complicated inner guts of the headrest. Only to find that the Brit's solution to his floppy headrest was not my solution. The spring in mine was firmly in place and operational unlike his which had broken free. What I did find was that the locking mechanism is actuated by a grey plastic sleeve on the LHS headrest pillar which slides up as the seat back is pushed into position. Mine does not, so there is something in the seat back which is not contacting this sleeve, to push it up and then trip the headrest latch. So if anyone knows how gain access to the workings of the seat back of the 3rd row seat so I can see what might be broken, I would be very grateful.