I read recently that in the event of a collision, the head rests on certain VW/Audi/Skoda cars can collapse downwards, thus leaving the passengers head with no protection from being thrown violently backwards. I checked my Skoda Octavia (VW-based) and found that although there are notch positions for the headrests, the rests can be pushed downwards with almost no effort at all, confirming what I had read.
Fortunately I have been able to overcome this fairly easily. Firstly I set the headrests the the best position (midway up the back of the passenger's head). Then I measured the gap between the bottom of the headrest and the top of the seat. I had some chromium plated tubing, and cut this to make pieces to fit the gap (only one per seat is needed). I lifted the headrests and slipped the tubing into position. It is now impossible to press the headrests downwards.
I think I have solved the problem, but if anyone thinks that this is not really the answer, please let me know.
John