On Sun, 4 Oct 2020 01:03:12 +0100, alan_m wrote:

Remove railings, cap stones and first course of bricks. Cover
exposed
top of wall with sheet lead that comes down the wall and over the
top
the roof covering fillet, both sides. Relay new clean bricks and
cap
stones, replace railings.

Could it be that the water penetration is where the cap stones have been
drilled through to fit the railings.?

Possibly but there are still the joints between the cap stones to
fail and let water in.

--
Cheers
Dave.