Might sound like a minor point (though in view of possible protection from whip-lash injury, perhaps not), but I'd like to get it right.

On my new Defender 110, 2005, one of the head restraints won't stay in position. At first was immovable, then local LR agent worked their magic. Now lifts clean out, and after 1 km of track driving, has moved itself down to flush with seat-top.

LR say "it can't be locked". How to keep it in position, then, in the position I want it?

Try good ole silver tape !

get a cable tie, pull it tight around the bar where you want the head restraint to stop, cut the loose end off, looks good to me