In reply to Relt99:
as per Matts instructions for rigging etc, tie a knot (big OH will do) underneath the gri gri whenever stopped.
the wire brush is ok if the rock is hard but depends on rock, don't use one of softish sandstone.
take a selection of brushes (from plastic washing up brush) through to wire. You may have to dig out some earthy bits so a nutkey (can damage) or wooden spike
make sure its not an SSSI or you are not trashing any rare stuff/nesting birds ....
wear old clothes, gardening gloves might help as well