I'm trying to find the name of an old Black and white movie I once saw. It is an English film about a doctor who discovers his wife is having an affair with an american man. So he kidnaps the man and locks him in an cellar chained to a wall with a circle of chalk around the perimeter so the doctor can visit him with food without the american being able to reach him. The doctor plans to fill a bath tub with acid and kill the american and then dissolve his body so there is no evidence. He does this slowly by filling a hot water bottle with acid everyday after work and filling up the tub. He is eventually undone when a detective notices the doctor is using american slang in his speech which he has picked up from his long conversations with his prisoner.

Does this ring any bells for anyone. It's a great film but it was pretty old and I can't remember the title or the names of any of the actors.

The Hidden Room aka Obsession (http://www.imdb.com/title/tt0041460/)