37 year old year old engineering technician Nicholas Hunter from Rodenhurst Road, Clapham pleaded guilty to manslaughter on the grounds of diminished responsibility.
In an unprovoked attack he used a pair of scissors and stabbed 49 year old James Fiennes several times in the chest in a tapas bar in Mortlake last April.
The victim was unknown to him.
Mr Fiennes, who is a cousin of the explorer Sir Ranulph Fiennes, died later in hospital.
His widow Caroline said in a victim impact statement:
He was kind, considerate, generous, funny, thoughtful, spiritual, intelligent and above all he was loving. He knew how to love. Anyone and everyone who met James described him as a gentleman in every sense of the word.
Old Bailey Judge Gerald Gordon told Hunter:
What you did was on any view utterly appalling and totally devastating for his family, as his widow’s personal impact statement makes movingly clear.
Detective Chief Inspector Simon Ashwin from the Homicide & Major Crime Command said:
This was a completely unprovoked assault on Mr Fiennes who was in London on business and was on his own working on his laptop in the pub before he was attacked and killed by Hunter.
Our thoughts and sympathies are with his family at this very difficult time.
Hunter has been given a hospital order for treatment in a secure facility without limit of time under the Mental Health Act.