I did manage to fix this, but it was a painful process
I had to remove the rear seats, which is difficult do to the bolts that are used, but I managed it eventually. I then removed the headrest and seat cover from the broken seat, again, this isn't particularly easy. You don't need to remove the cover completely, just unzip the and pull it up as far as you can to expose the catch mechanism for the headrest at the top. The catch is triggered by a cable that runs to the bottom of the seat and I think this can become unattached, or the cable can snap. In my case the catch just appeared to have seized up. I lubricated it well and made sure everything was operating nice and smoothly then reassembled everything.
Took half a day and a few grazed knuckles to fix, but the problem hasn't re-occurred
JA:
Look down the base of the headrest pole on mine it was the one closet to the center of the car. Find the pole with the spring running parallel to the headrest, just to the inside of the spring, towards the center of the seat, is the cable. The tip of the cable has a small nub on it. Seems when you lower the seat a metal piece, slightly smaller than the size of a dime, slides down the cable. When you raise the seat the piece of metal is suppose to ride back up the cable to let you manually prop the head rest back up. Chances are the metal piece to stuck about 1/2 inch down the cable. Now that you have located the tip of the cable spray WD40 on it. Open and close the seat a couple of time to get the lubricant down the cable and on to the piece of metal. The piece of metal will rise up to the tip of the cable and enable you to lock the headrest back into place.
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