No making good needed with a diamond corer. They break out very cleanly.
Well worth the price difference. Especially useful when the outside is not
easily accessible.
e.g. boiler flue for a 2nd floor flat.

I guess from this that you have installed a boiler flue in a 2nd floor
flat? I have a really similar problem - I want to put a vent pipe
though my kitchen wall (2nd floor flat) for a cooker hood extractor.
I'm happy to hire a diamond core drill to make the whole - but have no
idea how to put a cap over the vent pipe on the outside of the wall.

If I dont, I guess that water has a entry point into the render on the
outside of the wall - which doesnt seem like a good idea.

Any ideas?

Will