A drug addict has been jailed for torching their home and endangering his life after being invited to live with a Liberal Democrat councillor and his partner in an open relationship.

Dunstan Thomas was sentenced to four years in jail this afternoon at Croydon Crown Court for endangering the life of Councillor Hamish Pollock after setting light to his home and then engaging in a tense stand-off with police as the building blazed.

The court heard that Thomas, 41, was homeless and unemployed when he moved into the Sandy Lane South home and began a sexual relationship with the councillor, a situation apparently accepted by his live-in partner Ambrosio Reyes.

Speaking for the prosecution Stephen Hopper told the court Coun Pollock was woken at about 3.10am on June 11, by the fire alarm and smelt smoke.

Coun Pollock, who has represented Carshalton Central since 1990, ran downstairs to the kitchen where he saw Thomas had set fire to a wicker chair and a cardboard box.

As he put out the fires in the kitchen using buckets of water, Thomas rushed upstairs and set fire to clothes and the duvet in the main bedroom using a lighter.

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The court heard Coun Pollock, (pictured above) arrived upstairs to find Thomas by the bedroom cupboard armed with a knife he had taken from the kitchen so left the house and rang the police and fire brigade.

Thomas then locked himself in the bathroom and ran water from the bath tub and sink and covered the room with bathroom products intending to "wreck it," said Mr Hooper.

Firefighters did not enter the building aware that Thomas was armed and police had to force the bathroom door open.

Thomas lunged at an officer, who was protecting himself with a shield, before being subdued and arrested.

He later told police he had taken methadone 20 minutes before he started fires in the kitchen and afterwards felt an "electric feeling" and wanted to kill himself and had effectively forgotten that others were in the house.

During questioning Thomas said he had taken the knife to use on himself and was suffering from depression as well as from alcohol and drug addition.

Defence lawyer, Callinan Clodaghmuire, said: "I have never seen a case like this.

"It was not arson caused by someone who wanted revenge, but by someone who wanted to hurt themselves."

She added that Coun Pollock had written letters to Thomas in prison, had expressed an element of forgiveness and that it is anticipated that he will visit Thomas in prison.

Delivering the sentence Judge Stephen Waller said to Thomas: "You demonstrated a huge breach of trust that the householder placed in you, who showed great consideration when you were homeless."

Thomas was sentenced to serve four years in jail for committing arson with reckless intent to endanger life.

He had previous pleaded guilty to a charge of arson.