Sir Walter Gavin Gilbey, 68, ‘hurled’ his mother, Lady Elizabeth Gilbey, to the floor of her home in Pulborough on August 26 last year ‘in a moment of anger and frustration’ causing significant bruising to her chest.
Finding him guilty, magistrates in Horsham fined him £339, ordered him to pay £625 costs with a £33 surcharge and £200 compensation to be paid to his mother.
They also ruled that he must not contact his mother for a month with a restraining order placed on him for that time.
Following the initial period any contact must be supervised by her housekeeper or carer, the magistrates said.
Earlier they heard how Sir Walter - heir to the Gilbey gin empire - had snapped his mother’s walking stick into ‘several pieces’ during the assault before she fled to a neighbour’s house.
Eton-educated Sir Walter, dialled 999 immediately after and said his mother had gone ‘crazy mad’ making ‘wild assertions’ to the neighbours.
He claimed the incident was a lie concocted by his sister, Lady Camilla Frederick, over a family feud for control of Lady Gilbey’s estate.
But magistrates said his account was ‘not truthful’ after finding him guilty of one count of assault by beating.
Court chair Lesley Overington said: “In a moment of anger and frustration he intentionally assaulted Lady Elizabeth by grabbing her arms, pulling her out of the chair and he flung her to the floor face forward.”
She told the court: “Sir Gavin’s evidence was considered and not truthful.
“His answers to cross-examination were often evasive and off-point.
“After the incident and during his 999 call to the police he was advised to redial 999 for an ambulance, or phone 111.
“He did neither, despite his mother having gone to the neighbour’s house, where she was screaming, shouting and making accusations.
“We don’t accept he was calm during the incident. We find he did break the walking stick, as described by Lady Elizabeth.”
Ms Overington added: “We heard live evidence from Lady Elizabeth, whom we found to be credible on the issue of whether her son assaulted her. She was very clear about the details of the assault.”
Ms Overington also said evidence from Lady Gilbey’s GP, Dr Tim Fooks, was consistent with her account of the attack.
During the trial a photo was shown to the court of a large blackened bruise Lady Gilbey suffered to her chest during the assault. Another photo of her cane snapped into several pieces was also shown.
During the trial Lady Gilbey said she argued with her son in the sitting room of her countryside home in Pulborough before he returned and attacked her.
Giving evidence via video link, she said: “He snatched the stick from me and broke it into five pieces. He grabbed my arm and flung me on the floor and I landed on my face.”
She added: “He just stood over me and then I got up. I was so scared I ran into the neighbour’s house for protection.”