The painted stones having enjoyed a nights drying time, I gave them a wash with Games Workshop Agrax Earthshade to bring out the mortar lines. Simply washing the stuff over everything and then wiping the worst off the stone faces worked pretty well.
Left to dry, the effect becomes more subtle, but I still felt it was a little strong. A dry-brush with Sandstone out to tone it down I decided and carried this out on a corner of the wall for the photo.
Now I think it's gone too far and prefer the earlier version. Hopefully a second wash will sort this, then I'll do the rest of the building.
Practise on a hidden area is good advice!