No, it's real!!
The kidnap scene was filmed in Hampstead, very near to where I was living at the time. I've no idea where the big house where he held her is, though. Loved the two-tier underground basement.
Story has it that the director William Wilder picked Stamp and Eggar for the roles to reflect reality, as she had rejected him in real life when they were both drama students. Mentioned this to a girl friend a while ago. Her response:-
'What a silly bitch!'
Okay, add Christian Bale in 'Straw Dogs' to the list!!