Hi
If the tiles are really that bad you can use either sugar soap them and scrub between the tiles or use Spirit of Salts which is basically 32% hydrochloric acid.
Please be careful with the acid, it STINKS! and is acid afterall.
You can fill the bath with water and put the acid in but you have to be careful as it can damage the enamel or paint.
Hope that helps
Regards
Martin