I answered the question about Christchurch Festival myself tonight.
I remembered that a friend sent me a copy of the 1972 BBC film they made about the festival (part of which I am singing with Helen Woodall in the Old George courtyard) - and it was the 3rd Festival so the title says - so 1970 was the first.
Alan white - who also appears in the film (we were on the same Morris team) was right.

Tim Radford