I don't know about floors but I spray my bathroom wall tile grout with milton's sterilising fluid then clean with a toothbrush. It takes a while but the grout comes up lovely.
I've got karndean in my kitchen & I've found it excellent. Got some left over for the bathroom, which I'll do when I get a roundtuit. As for the grouting, I'd use bleach & a nailbrush. Wear all the necessary safety equipment;goggles are a must!!