A pair of teenagers who started a fire in a derelict house, causing the death of a homeless squatter, have had their sentences cut at the Court of Appeal.

But senior judges dismissed a bid by the 17-year-old girl and 16-year-old boy, who torched a duvet in the former Sea Cadet centre in Waddon in 2013, to have their manslaughter convictions quashed. 

Sylwester Mendzelewski, 35, was sleeping in the basement of the house, in the Waldrons, when the fire took hold on June 10.

He died of smoke inhalation after the fire spread to discarded tyres and thick, acrid smoke filled the room.

The teenagers, who cannot be named because of their age, were sentenced to three years detention on August 18 last year after being convicted of manslaughter and arson.

 But their lawyers appealed the convictions and sentences, arguing Judge Warwick McKinnon had not correctly directed the jury at Croydon Crown Court on the level of intent and foresight required for manslaughter.

From August 2014: Teens behind bars for causing death of homeless man in Sea Cadets arson

From June 2014: Police treating fire death in squat as suspicious

At the Court of Appeal hearing on February 17, three senior judges upheld the conviction but allowed the sentencing appeals. 

Lord Thomas of Cwmgiedd, the Lord Chief Justice of England and Wales, said the convictions were not "manifestly excessive" but should be quashed because the 17-year-old girl was making "very significant progress" in the youth offenders institution where she is held. 

The girl will turn 18 in May, when she would be moved to a jail for older offenders.

Lord Thomas said: "It was plainly in the interests of rehabilitating her and protecting society from the commission of further offences by her that she not be moved, given the very significant progress made and the obvious risks to her continued progress by the different environment of an institution for those over the age of 18."

He reduced the girl's sentence to a 24-month detention and training order and gave the same sentence to the boy to ensure parity. Half of the sentence is served in detention, meaning they will be freed in August.

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Lord Thomas of Cwmgiedd, one of the most senior judges in the country, cut the teenagers' sentences

Detective Sergeant Adam Manley, of the Metropolitan Police's Homicide and Major Crime Command, said last year the teenagers were "not hardened criminals but simply two individuals who did not think through the consequences of their actions". 

He added: "Both have shown immense remorse but still have to live with the knowledge of what they did and the devastating impact on the lives of others."

Polish national Mr Mendzelewski had been squatting in the house with three other people for seven months before his death. His body was found by firefighters. 

The teenagers admitted lighting the fire, but said they had not realised anyone else was there. 

In the days following the fire there the boy, then 14, talked on Facebook to a friend who filmed the arson but later deleted the footage. 

The  girl also tweeted a friend, saying: "Oh my god what have I done".