The best I've found so far are the ones that fit on an angle grinder. http://www.tradetiler.com/tile-cutt...dry-gres-m14-diamond-tile-drill-core-6mm.html You can drill accurately with 6 and 8mm, no need for a guide as the bit doesn't slip. Haven't used the bigger sizes so far...may need a guide, but easily made from a bit of thin ply and clamped on. Go gently and keep them cool with a wet sponge. If you're really careful, you'll prob get about 15 holes out of one, also depending on the grade of porcelain, so about £1 per hole. May seem expensive, but worth every penny IMO. These make a PITA job so much easier.