In reply to Sam Beaton:
I used some horrible bleach type spray on mine (similar to Dettol Mould and Mildew remover).
I lay it flat. Sprayed it, wiped/scrubbed it. Sprayed it, left it for a while, wiped it and cleaned it. Turned it over and did the other side.
Didn't seem to have any bad effects on it for me. It is nice and shiny and yellow again.