The Natural Philosopher wrote:

No. And therein lies the problem. They wander like buggery until you get
them started, then they stay in the groove

One way to sort that is to make a 6mm hole in something and use that as a
jig pressed against the wall.

I've been Googling this. With the larger sizes the recommended technique
appears to be to approach the surface at a 45 degree angle, let the drill
bite in just one spot, and then slowly move it to 90 degrees with the whole
of the drill in contact.

Maybe the old trick of putting masking tape or similar on the glazed surface
would help?

I might see if I can find a scrap tile or two to practise on - but not too
much, to avoid blunting the drill.

Thanks for the info.