Grout stains

[terrya]

What effective method can I use to remove the black marks on the white grout between the mosaic tiles on my shower floor. Thanks in advance

[Mosaic]

You can get a grout cleaner that is in effect a very thick bleach - apply it to slightly damp grout and go out for the day, scrub off when you get back.

[Wak]

Dettol Mould and Mildew Remover?