In my bathroom, I have standard cream ceramic tiles on the wall above the bath, but the once dazzilingly white grout is now a rather repulsive shade of dirty yellow. I've tried scrubbing it, but to no avail - would it be a big job to bring it back to pristine condition?

I've heard of 'grouting pens' you can use, but, well, the name just sounds a bit gimmicky really, so I was wondering what all you experts out there would do in my situation. Does this require a whole weekend, or can I tie it up in twenty minutes?