[Arran]

Hi;
  I think that the grilles in those are made out of polystyrene plastic, so a good bath in warm water and a little laundry soap should help, you will have to remove the grille from the cabinet naturally. It won't remove the scratches of course but it should make them less noticeable, unlike Bakelite polystyrene won't go dull from standard cleaning products. The only way to remove scratches in plastic is by wet sanding with extremely fine sandpaper, in a succession of steps into finer and finer grits, followed by by a plastic polishing compound such as Novis or one of the polishes used for auto paint.
Regards
Arran