In reply to zoobizooretta:
I did look at the hilti stuff, but it woukd cost thousands to cover my wall with those fixings. I know that covering the wall in plywood is the easiest option, but the wall is a nice feature in the garden and it would be a shame to do that. I've uploaded a photo of it to my ukc account (awaiting approval) so you can have a look at what I'm dealing with.
The holds on the wall are held with expansion bolts and seem fairly solid, but the mini jugs just exert too much force and crack the rock. I'm chisseling the rocks flat behind the holds so that they're attached to a flat surface.