A businessman imprisoned his 84-year-old mother, pulled plastic bags over her head and attempted to blackmail her out of £120,000, a court heard.

Julian Davies, 47, of Hamilton Road, north Oxford, was found guilty at Oxford Crown Court on Tuesday of falsely imprisoning, blackmailing and maliciously causing Thelma Davies grievous bodily harm on October 15. He was cleared of intentionally causing grievous bodily harm.

Davies forced his mother to write letters to family members confessing she had cost him £120,000 by making a malicious phone call to him at work in 1996. He put plastic bags over her head, threatened to kill her and caused arm injuries when he tussled with her.

Daniel Bigwood told the court he saw Davies arguing with his mother outside his house, and saw blood on her shirt and wrists. Former merchant banker Davies, who told the court he was the brains behind an Oxfam credit card which raised money for the charity, said moments before his mother had come to his home, he had read a letter from her full of diatribes.

He said: "She has a habit of saying strange things and then recants. It seems to be an aspect of her character to try to taunt me."

He claimed his mother became mocking so he put plastic bags on her head to resemble Easter bonnets, to make her realise how childish her behaviour was.

Davies is detained at a psychiatric hospital. The case was adjourned for pre-sentence reports until June 7.