I found that in order to get the sagging lining to stick to the backing board, I first had to remove as much of the old crumbly foam as I could from both surfaces (using a stiff brush).
I then painted the board with PVA glue to stabilise any remaining foam and dust, left that to harden off a bit, and then stuck the fabric back on the board using upholstery adhesive (Evo Stick, I think)
It has stayed up for a year now without sagging again.
Oh - and warm water with a small amount of soapy detergent seems a good bet for cleaning.
Andy
(Still waiting for his Landy to come back from the garage after some muppet drove into the back of it!)