Tips please on how to accurately drill into pebbledash with (super tough) sand-lime block behind.

[Aop]

I want to mount a square plaque by the door and it needs to be exactly horizontal so as to not look like a dogs breakfast. I take it I can chip a pebble off if there's one exactly under the drill. Any other tips to make a neat job of it gratefully received. Is an ordinary masonry bit the correct tool to use?

You could use Gorilla glue. It expands into gaps and will certainly be strong enough.

I suggest this because I've got pebbledash and have never managed to drill accurately into it.