Good luck with that one Martin - a notoriously difficult set to trace in our experience - I'd grab anything on offer if I were you if you seriously want a full set.
We had volume one for years and finally stumbled on a full set when a collector friend sold off many of his books to finance the issuing of CDs from his collection of field recordings.
A set well worth having (and having part sets is not unlike waiting for the other shoe to hit the floor in the upstairs flat!)
Jim Carroll