Thank you all for your input, everyone.
So to summarise:-
They probably existed in Britain around the first century BCE because some Romans called them Druids and because Wiki says so ;o)
The hard archaeological evidence for their existence is based on 3 graves in Essex which (may have) contained at least one druid and dated circa 60-40 BCE.
So it is unlikely then that they were in this country before the late IA and even more unlikely that they had any influence the later stages of Stonehenge or any other of our pre-IA structures?
I will have to have a read of that book Ocifant :o)