In article ,
"Rick Hughes" writes:
1 part cement 4 parts sand ......... use plasticiser in mix if you have it
(or add some fairy liquid to the water - lowers surface tension and entrains
air)

I would suggest wire brushing well before hand and damp down surface and
mating face of coping with a mix of unibond & water.

Mix a little Unibond PVA into the mortar mix too for the coping
stones (or top line of bricks on a wall), which will add a little
tensile strength to the mortar and make it harder to simply lift
the stones off.

--
Andrew Gabriel