A teenage gang have been convicted of killing a mother-of-nine by pushing a firework through the letterbox of her home after a string of bullying attacks on her teenage son.

Mary Fox, 59, died in the fire in Bodmin, Cornwall, on Bonfire Night last year when the Silver Dart rocket was thrust inside the house to "frighten" the family.

Her son Raum, then 17, escaped through a window of the blazing council home on the Berryfields estate, but his mother inhaled a fatal amount of fumes.

Owen Hewitt, 18, and Samual Luckes, 17, convicted of manslaughter and arson earlier, were sentenced to seven years in a young offenders' institution.

Ryan Croft, 18, of Wallace Road, Bodmin, was sentenced to five years.

Croft, who lived a few doors down from Mrs Fox, admitted manslaughter and arson at a hearing in February.

His friends Hewitt and Luckes denied any involvement with the incident.

But yesterday, three weeks into the trial, and after almost 17 hours of deliberation, the jury found Hewitt and Luckes guilty of both charges.

The teenagers showed no emotion as the jury of six men and six women returned majority verdicts, while members of the defendants' families gasped and wept.

Sentencing, Mr Justice Field described the three gang members' behaviour as "yobbish", and that they had inflicted "a vicious form of bullying" on the Foxes.

The judge added: "You all decided to put a firework through the letterbox to frighten the Foxes.

"You didn't intend to harm them, but this was a grossly reckless, dangerous and irresponsible thing to do."

Outside the court, Devon and Cornwall Police Supt Craig Downham agreed with Mr Justice Field's sentiments, adding: "This was a very difficult but thorough investigation which has had a significant impact on the Berryfields estate and the wider community of Bodmin.

"The community of Bodmin, especially on the Berryfields estate, has gone through tremendous upheaval following this awful incident.

"Thankfully, however, cases like this are rare in Cornwall and the Isles of Scilly, but when they do occur we will continue to do our utmost to ensure offenders are caught and then brought to court."

The court heard how all three teenagers were drunk and had smoked cannabis on the night of the fire, and were setting off more than £125 worth of fireworks, bought by Hewitt's step-father, Lee Hewitt.

The teenagers routinely blamed each other for the tragedy, Hewitt and Luckes saying Croft put the firework through Mrs Fox's home after having the balancing stick snapped off, in order to make the firework "skip" around, causing maximum destruction.

Croft was deemed an "unreliable witness" and was not called to give evidence to the court during the trial.

However, during sentencing, Paul Dunkels QC recalled Croft's police interview in which he first confessed involvement.

Reading the statement, Mr Dunkels said Croft told police: "I am sorry for messing you around, I was just scared when I heard it called 'murder'.

"I have tried lying my way out of it, and tried lying my friends out of it.

"I held it (the letterbox) open while Sam (Luckes) pushed the firework through and Owen (Hewitt) kept a look out. They did it to make them (the Foxes) jump."

Hewitt and Luckes described the incident as "a prank", aimed at Raum Fox, who Croft and others used to bully. They said they were unaware of what Croft apparently did when he "disappeared" into the Fox's heavily-barricaded garden.

Witnesses told how the youths had been setting fireworks off at various locations in Bodmin, firing them at friends and a mother and baby.

They returned to the scene of Mrs Fox's fire-ravaged home at about 10pm, less than three hours after the firework attack, where they stood and watched as emergency services battled to control the blaze.

News of the devastation had reached the teenagers earlier that evening while they were walking through Bodmin.

Croft was said to have told one witness: "I think we might be in trouble".

Police, who approached the gang, said one of the defendants was "subdued, almost tearful", while another was "on edge".

Prosecuting, Mr Dunkels described the incident as "a joint venture".

He said: "They were in it together – it doesn't matter who did what precisely.

"Each of them played some part and is responsible for what happened."

Hewitt, of Bodmin Foyer, Kestenenn, and Luckes, of Rhind Street, both have a number of previous convictions and were sentenced to seven years in a young offenders' institution for manslaughter. They were each also given a three-year sentence, to run concurrently, for arson.

Croft, with a previous conviction for criminal damage and battery, was given a five-year sentence for killing Mrs Fox, with two-year concurrent sentence for arson.

All defendants will have the 196 days spent on remand deducted from their sentence.

A Cornwall Council statement said the Foxes were known to the authority, but "were not regarded as being at risk".