A Dirty Knight's Work (1976)

A group of British aristocrats, who call themselves "Knights of Avalon", isn't content with the system of justice and executes judgment themselves. Instead of just killing the people they believe guilty, they give them a chance in traditional combat. When one day, Sir Edward Gifford (Peter Cushing) witnesses their sessions, they have to remove him too. His son and heir, Sir John Gifford (David Birney) from America, starts to investigate with help from Ms. Marion Evans (Barbara Hershey) and Colonel Bertie Cook (Sir John Mills) from Scotland Yard.