On Mon, 05 Oct 2020 13:15:08 +0100, Pamela 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.

All very true and something to do in spring. Meanwhile the wall needs
sealing as rain water is entering...

As a "keep the wet out 'till the spring(*1)" solution I'd just get
some reasonable weight poly sheeting and a few bricks(*2) and simply
cover the cap stones, bricks along the edges every 2 to 3 feet. With
the sheeting an inch or two wider each side so water drips off onto
the flat roofs. The delux version would have a batten along the line
of the railings to enforce a fall on the sheeting and allow a bit of
ventilation underneath. Super delux tape any joins between sheets
rather than just having an overlap.

Trouble is we don't really know where the water is getting into the
wall. It could be through the joints in the cap stones or beteen the
wall and roofing fillet or both... try just the capstones if that
doesn't have much effect it's easy to put a bit sheeting under it and
down to the roof (weighted down of course) to fully protect the wall/
fillet as well.

The caps stones do have a drip groove underneath the overhang don't
they?

(*1) I'm assuming spring 2021, not '22 or '23... B-)

(*2) Such as scrounged empty feed bags from local farmers opened out
and spare rocks that are lying around or even a logs from the wood
pile.

--
Cheers
Dave.