AlexW wrote:
The coping stones that sit on top of the wall have about a 30mm overhang
which is not sufficient for the water to drip onto the ground. Nor did
the builders put a drip (groove) on the underside (as specified!).

Then you get the builders back to put a drip groove ... You haven't paid
them yet have you?

You might also want a drain (pebbles and perforated tube) along the
outside of the wall

Owain