Kevin,

You could very well be right. Here's a list of credits for the movie. Can't say I recognise anyone who might have been our prototypical folkie.

John Mills ... Phillip Davidson
John McCallum ... Supt. Bob Lowther
Elizabeth Sellars ... Fay Lowther
Eva Bergh ... Ilse
Geoffrey Keen ... Craig
Michael Martin Harvey ... Jackson (as Michael Martin-Harvey)
John Chandos ... Boyd
John Slater ... Pewsey
Thora Hird ... Mrs. Pewsley
Vida Hope ... Alice Gedge
Harold Lang ... Boyd's chauffeur
Mary Mackenzie ... Gladys
John Glyn-Jones ... Gedge
John Horsley ... Bletchley
Fred Johnson ... Driver
Laurence Naismith ... Hasbury
Peter Jones ... Fisher
Christopher Beeny ... Mickie
Ernest Clark ... Proscecuting Councel (uncredited)
Henry Edwards ... Judge (uncredited)
Arthur Mullard ... Policeman (uncredited)
Denis Shaw ... Shaw (uncredited)
Julian Somers ... Delaney (uncredited)
Russell Waters ... Scotson (uncredited)

Produced by
Earl St. John .... executive producer
Hugh Stewart .... producer

Original Music by
William Alwyn   

Cinematography by
Harry Waxman   (photography by)

Film Editing by
Gordon Hales   

Casting by
Weston Drury Jr.   (uncredited)

Art Direction by
Alex Vetchinsky   (as Vetchinsky)

Costume Design by
Joan Ellacott   

Makeup Department
George Blackler .... makeup artist
Biddy Chrystal .... hair stylist (uncredited)

Production Management
Denis Holt .... production manager

Second Unit Director or Assistant Director
Robert Asher .... assistant director
David W. Orton .... assistant director (as David Orton)
Christopher Sutton .... third assistant director (uncredited)

Art Department
Bert Gaiters .... property master (uncredited)
Brian Herbert .... chief draughtsman (uncredited)
Arthur Taksen .... set dresser (uncredited)

Sound Department
Gordon K. McCallum .... sound recordist
Winston Ryder .... sound editor
C.C. Stevens .... sound recordist
Bill Daniels .... dubbing crew (uncredited)
Basil Fenton-Smith .... boom operator (uncredited)
John Salter .... assistant boom operator (uncredited)
George Willows .... sound camera operator (uncredited)

Special Effects by
Bill Warrington .... special effects (as W.Warrington)

Camera and Electrical Department
James Bawden .... camera operator
John Alcott .... clapper loader (uncredited)
Jack Atcheler .... focus puller (uncredited)
Ian Jeayes .... still photographer (uncredited)

Costume and Wardrobe Department
Dorothy Edwards .... wardrobe supervisor: women (uncredited)
Bob Rayner .... wardrobe supervisor: men (uncredited)

Editorial Department
Derek Armstrong .... assistant editor (uncredited)
Anthony Harvey .... second assistant editor (uncredited)

Music Department
Muir Mathieson .... conductor

Other crew
Arthur Alcott .... production controller: Pinewood Studios
Barbara Cole .... continuity
Teresa Bolland .... production secretary (uncredited)
Ken Green .... publicist (uncredited)
Bill Kirby .... location manager (uncredited)