Professor Chadwick’s 1968 Ethel M. Wood Lecture is available on-line, thanks to the author’s kind permission a couple of years ago:
The Rev. Prof. Henry Chadwick, The Enigma of St. Paul. The Ethel M. Wood lecture delivered before the University of London on 27 February 1968. London: The Athlone Press, 1969. Pbk. ISBN: 0485143135. pp.22.
It used to be said that you had to study patristics for at least 50 years to be considered an expert. If that is true then men like this will prove hard to replace.