It is also a radio play

Here is a BBC link.

http://www.bbc.co.uk/programmes/b007jyxz

It is currently available to listen to via The internet at least for the next six days and will probably be repeated in future.

Excellent. The information in these IMDB comments and Boards adds to it especially the stuff about Michael Redgrave. Presumably, although in the Media all my young life, Burgess, Philby, Maclean and later Blunt, were mysterious shady characters, to some both, in Russia and elsewhere, they were actual people who maintained real life relationships independent of their political and employment connections, just like the rest of us!