NY wrote

I want to fasten a couple of wooden fence posts, about 1.5 m high, with a
fence panel fastened between the posts, on top of a brick wall that is
capped with stone slabs.

I wouldn’t do it myself. Brick walls arent that strong
to sideways forces on the fence on top of the wall.

I'd have steel poles in the ground with the fence
bolted to those above the height of the brick wall.
That would look fine with the fence on the wall side
of the steel poles up against the brick wall with
some form of spacer if necessary for appearance.

The brick wall appears to be made of two bricks laid side by side - I
can't be certain without removing a cap-stone, and they are firmly
mortared in place. However the width of the slabs is 270 mm, which
suggests two courses of brick side by side. The slabs are 25 mm thick.

Sketch: https://i.postimg.cc/cJd877jn/fence-posts.png

The main problem will be the amount of sideways leverage that there would
be on the posts if a wind like Storm Ciara/Dennis blows on the panel.

I have seen mounting brackets such as
https://www.diy.com/departments/bloo...1628475_BQ.prd
which have four holes.

What is the best way of drilling through a stone slab without it cracking?
I presume the best way to anchor the bolts is to use expanding metal
bolts. Is it best to try to make the fixing expand into the brick, and not
also to expand into the stone for fear of cracking it.

It would be possible to drill a couple of holes horizontally through the
wooden post, sideways into ends of bricks that make up piers at the end of
the wall, to provide additional fixing for the posts, though these bolts
would only be 1 brick above the level of the slabs.