Thanks for that Nick, I was unaware that HCH was one of the nasties.
Have a read of my previous post 'Clematis lost all their flowers' - just
above the HCH post. The reason I believe ants are causing the damage is
because I have seen a large number of ants crawling over the flowers (what
flowers are left!) and there is a large nest at the base of the plant.
My rockery is swarming with ants & woodlice & again there are small holes in
all the petals of all the flowers.

Jo