Afternoon,
This has been discussed a few times, and since I've just managed to do this successfully, I thought I'd share the experience! I was in B&Q and noticed that they now stock a "Ceramic Tile Cleaner" in a trigger spray bottle, next to the Plasplugs stuff. It seems like a less strong version of heavy duty floor tile cleaning fluid, but it's cheaper and the trigger spray makes it quick to use.
I started off by spraying an area of just under 1 square metre (one squirt per 200mm x 200mm tile). I left it for about a minute and then used a tiling sponge (absorbent) to wipe the surface clean. Areas where the grout glaze was a little thicker needed some extra encouragement, but the majority came off just by wiping. For stubborn bits I used a small section of pine batten to rub the surface.
The fumes are quite strong, so I would recommend good ventilation, and you definitely need to use rubber gloves.
It took about 10 minutes per square metre, and there was no effect on the grout between the tiles, apart from possibly making it slightly cleaner.
It's worth noting that I wouldn't have had a problem in the first place if I hadn't used white tiles. When I tiled my kitchen in black tiles, I had no trouble making sure that the tiles were clear of grout on the surface by the time I had finished, because I could see anything left. In the bathroom (large white tiles), I found it *very* difficult to tell whether the surface is clear of grout.
Hope this helps someone! Al