Thanks to all.
I will try the cow dung (Is horse dung as good?) but if that doesn't
work...
One of the walls is forming a raised flower bed. What sort of plant
could i use that could be planted in the bed and grown over the wall
to cover the face?

Regards,

Rich

"David Hill" wrote in message ...
The old way of getting moss etc to grow on new stonework was to paint with a
solution of Cow dung, failing that a slower way was mil or as said Yoghurt,
but if your walls are south facing then none will work to any degree.
When I built a bridge over our stream I left some joints open to the backing
soil and planted cliff break fern into them, all took but none have any
sunlight and are damp most of the time.
No fern will be able to root through cement to find soil.