I am trying to remember the name of a movie (seventies I think) where a US guy is kidnapped and taken to Israel to stand trial on war crimes as a concentraiton camp commander. He plays the part in great detail only for the prosecution to realise they got the wrong guy. I can't remember how it ends. It had a big effect on me as a kid and I can't remember what it's called. Any help gratefully appreciated. Thanks.

Gratefully even. Doh...

I think your looking for The Man in the Glass Booth - here's a snippet that matches your description:
"...Now a rich industrialist, he lives in luxury in a Manhattan high-rise. He banters with his assistant Charlie, often shocking him with his outrageousness and irreverence about aspects of Jewish life. One day, Israeli secret agents kidnap Goldman and take him to Israel for trial on charges of being a Nazi war criminal..."
The Man in the Glass Booth - Wikipedia, the free encyclopedia