I don’t have a copy of it, but it occurs to me that you might find something of interest in ‘Camp 020: MI5 and the Nazi Spies — The Official History of MI5’s Wartime Interrogation Centre’ by Oliver Hoare (PRO 2000 ISBN 1-903365-08-2). To put it in context, it’s mentioned in an old thread which can be found at http://forum.keypublishing.com/showthread.php?99256-Comper-Swift-F-AOTP-%28ex-G-ACDS%29.