So the main dealers said my stuck and rattling headrest not covered under used car warranty, and I didn't cop it till over two weeks from sale. Also said they were sealed and not repairable. If I'm going to have to buy a replacement, then might as well have a go at fixing the broken one, so here goes.
With a pick tool I managed to extract the leather flap end which is pressed into the base between the bars at the front underside.
Front edge of base flap.
Peel back. Repeat for corner and side folds which are similarly pressed into the internal frame.
Pop out side button, use flat tool.
Slide the frame out of the cushion
Remove two Torx screws, and split the case from the bottom, hinging on the top, there are two click fit catches in the middle down a recess on the other side, you might have to prise these a little.
Bingo! Two loose springs and slides fell out, plus a few tiny bits of sheared plastic.
These it turns out are what retained the slides pressed against the springs under the bar. Not sure of its purpose, could be some kind of one use anti whiplash brake on the bars.
The plastic retaining lugs for this spring are also almost fully broken off, allowing the spring to rise up.
With the spring removed you can see the lug hooks hanging off.
The small broken pieces came from here, which would retain the two slide pieces against spring pressure held in the area above against the round protrusion. The other end of the slide was also retained by even smaller pin size pieces of plastic moulded on that protrusion which had broken off.
These slide devices seem to rub against the bars as you move the headrest fore and aft. With the retainers broken, the slides seem to wedge the bars under pressure from the spring, which may fall out leaving the slide loose and rattling against the frame.
I've glued a small crossbar of wire to replace the other hinge spring retainers, and I think I'll just glue the two slides in position without the springs, whatever their function was is not clear, but I'm guessing some kind of whiplash inhibitor.
Hinge spring retained by a piece of wire hot glued in place of broken lugs. The slides were glued in and the springs discarded.
Reassemble, replace on seat and test.
Slides forward and back perfectly, no rattles while driving.
With a pick tool I managed to extract the leather flap end which is pressed into the base between the bars at the front underside.

Front edge of base flap.

Peel back. Repeat for corner and side folds which are similarly pressed into the internal frame.

Pop out side button, use flat tool.

Slide the frame out of the cushion

Remove two Torx screws, and split the case from the bottom, hinging on the top, there are two click fit catches in the middle down a recess on the other side, you might have to prise these a little.

Bingo! Two loose springs and slides fell out, plus a few tiny bits of sheared plastic.

These it turns out are what retained the slides pressed against the springs under the bar. Not sure of its purpose, could be some kind of one use anti whiplash brake on the bars.

The plastic retaining lugs for this spring are also almost fully broken off, allowing the spring to rise up.

With the spring removed you can see the lug hooks hanging off.

The small broken pieces came from here, which would retain the two slide pieces against spring pressure held in the area above against the round protrusion. The other end of the slide was also retained by even smaller pin size pieces of plastic moulded on that protrusion which had broken off.
These slide devices seem to rub against the bars as you move the headrest fore and aft. With the retainers broken, the slides seem to wedge the bars under pressure from the spring, which may fall out leaving the slide loose and rattling against the frame.
I've glued a small crossbar of wire to replace the other hinge spring retainers, and I think I'll just glue the two slides in position without the springs, whatever their function was is not clear, but I'm guessing some kind of whiplash inhibitor.

Hinge spring retained by a piece of wire hot glued in place of broken lugs. The slides were glued in and the springs discarded.

Reassemble, replace on seat and test.

Slides forward and back perfectly, no rattles while driving.