[Arran]

You would pretty much have to remove the speaker and possibly the chassis first, remove the grille cloth which is glued to a piece of thin cardboard, first before attempting to clean it. In terms of what to clean the cloth with I think that some form of dry cleaning would be best, but many dry cleaners would need you to remove the cloth from the cardboard first. However auto parts stores sell perchloroethelyene in the form of brake cleaner in spray cans which is the same chemical used by dry cleaners, you can spray it on the cloth and then dab it with a clean rag or a paper towel until it dries and comes up better.
Regards
Arran