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. 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.