Try a diluted vinegar solution of about 1 cup of any vinegar (I use white) and 1 cup of warm water. Using one of the spongeback kitchen scrubbers, I find that it works quite well. It's best not to let the vinegar mixture set on the grout line to long... rinse it with warm, clear water.
Friends tell me there's a fairly new product, at least here in the U.S., called Sulfumic Acid that works quite well. It's apparently easy to work with without all the smell of Muratic Acid.
Additionally, I've picked up a little packet of tools at the DIY store that resemble Dentist's metal picks. They're very handy to even up grout lines and help remove the grout from rough areas of the tile, if they have any...
Best of luck!