I've got to admit - at the moment, these are links to other pages - mostly the wonderful resource which is IMDB. As soon as I get time, I'll put together something original (if anyone wants to do it for any one of the actors below, or anyone else who was in the film, please do it, and e-mail me the results - you'll get full credit, and my thanks!
Sidney Poitier (Mark Thackeray)
Judy Geeson (Pamela Dare)
Suzy Kendall (Gillian Blanchard)
Lulu (Barbara Pegg)
Christian Roberts (Denham)
Chris Chittell (Potter)
Adrienne Posta (Moira Jackson)
Gareth Robinson (Titch)
Lynne Sue Moon (Wong)
Anthony Villaroel (Seales)
Michael Des Barres ('Gypsy' Williams)
Faith Brook (Grace Evans)
Geoffrey Bayldon (Weston) - Probably better known, particularly to UK readers, as Catweazle
Patricia Routledge (Clinty) - Definitely better known to UK readers as Hyacinth Bucket!
Edward Burnham (Florian)
Rita Webb
(Mrs. Joseph)