On 17:23 5 Oct 2020, Andrew said:
On 04/10/2020 01:23, Dave Liquorice wrote:
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.

Also, there may not be a drip channel on the underside of the
overhanging lip of those slabs. All the rainwater pouring off the edge
will wick under and down the wall without a drip channel.

Window and Door external cills have a drip channel for a reason !.

Complete removal and the addition of a lead or copper flashing right
across the wall below would be the 'proper' fix.

A drip channel was put in with an angle grinder. I think the concrete(?)
slabs weren't correct for the job and may be porous. Hence the sealant
paint.

The amount of water this arrangement let in to the room below during a rain
shower was amazing. Until the true cause of the water was discovered, the
roof was tanked with fibre glass like a boat (on the near side in the
picture) but it didn't help.

https://postimg.cc/N2qjQ9kj