Needing to accurately drill six holes in the wall of our tiled shower area to fit a handrail and, finding that my 6 mm masonry bit had gone missing, off I went to our local DIY store. I was attracted by Wolfcraft kit of diamond drills and a drill jig like this:

I bought one and took it home, drilled the first hole which seemed to go well, measured the position for the next hole, set up the jig and drilled the second fixing hole – it was off position by about half the diameter of the 6 mm hole 
A quick investigation showed that the drill bit shank was 8.9 mm and the hole in the yellow guide part was 9.52 mm. Furthermore, when the drill is just in contact with the tile surface, only 8 mm of the shank is within the guide. Add to that the fact that the fit of the guide in the body is nearly as loose and the tip of the drill bit can move by about 6 mm away from the centre position. That's a bit much considering that the smallest drill in the set is 5 mm dia.
At least the diamond drills cut well sideways so I could finish the job by using the jig to start the holes by about 1 mm and then cutting sideways without the jig to correct the position.
Added to the To Do list – make a new guide from brass or bronze.
Russell