Oh boy - I've tried using polishing agents like Brasso (left a horrible mess), sandpaper, wet and dry, high and low speed sanding attachments, an aluminium oxide sanding disk and lastly an angle-grinder mounted high speed flap wheel. All to virtually no avail.
Has the cutting process hardened the surface of the tiles in some way ? Does anyone have any ideas of what else I can try ? I've heard of something called carborundum paper but never seen any (and B&Q have never heard of it) - anyone know anything about this ?