I recently bought some black (not polished) tiles from B&Q, ive used fastset as adhesive & Grout. I finished cleaning down & all looked well. When i went in to fit shower the following day, there was a lot of residue on the tiles. It looks like some of it may have penetrated the pores of the tiles, as no matter how hard i scrub (with heavy duty scourers) it doesnt make a lot of difference. when the tiles are wet it looks perfect, however when it dries the greyish colour starts to show in areas. My question is. Is there a liquid, paint on, wet look sealer, i can use to give it the permanent wet look. can anyone recommend

hiya what kind of tiles are they

You have used adhesive to grout with as well.. .. ok..

The tiles are slate effect porcelin

Try using a heavy duty tile cleaner available from most tile stockists. Follow the instructions on the reverse and you should be ok.
There are some excellent products available at Tradetiler.com Home