Every time I came back from a drive I noticed that there was a yellow powder on the seats and on the floor behind the seats.  It seemed to be coming out of the headrests, which were not very solid. Having removed one, and carefully pulled the underneath of the cover apart, I could see that the foam inside had turned to powder.  No doubt caused by 17 years of exposure to the sunlight in the summer driving!  The foam on the outside, in contact with the cover, seemed to be fairly intact.With the foam removed, I turned the cover inside-out and brushed off off the loose foam.  Most of it came away fairly easily.

The more stubborn foam that seemed to have become stuck to the cover was washed away under the cold tap, with a little bit of scraping with a knife blade and a toothbrush.

The cover was fine after drying out.  It was showing signs of previous DIY repairs with what seemed to be the remains of 'Copydex' adhesive around bottom opening. 

There was a small amount of foam stuck on the metal frame of the headrest.  After I had removed the pieces of foam that were positioned between the holed metal plate and the support, I soaked the whole lot in warm soapy water, followed by light application of a toothbrush, fine flat file and sandpaper.

All nice and shiny, ready for drying out on top of the central heating radiator!