Thanks for the pointers everyone. I managed to get a copy from Biblio
for £3.99 (plus the same again for 2nd class post). The copy I
received was a hardback in non-mint but generally good condition. It
seems to have belonged to Cornwall County Library, but was last taken
out in 1980 and is marked 'withdrawn' so has probably found its way
legitimately on to the used market. It's a 3rd edition dated 1969 - I
guess I could have asked here what was the latest available edition
but presumably 1969 is not too out-of-date for most of the issues that
it covers.

JGD