The owner of a fireworks company and his son were jailed today after being convicted of the manslaughter of two firemen killed in a huge blast at their family-run company.

Martin Winter, 52, was handed a seven-year jail term and his 25-year-old son, Nathan Winter, was sentenced to five years for the deaths of retained firefighter Geoff Wicker, 49, and support officer Brian Wembridge, 63.

The pair, both long-serving members of East Sussex Fire and Rescue Service, died in the blast at Festival Fireworks UK Ltd at Marlie Farm in Shortgate, near Lewes, on December 3, 2006.

Footage taken by Brian Wembridge of the blaze

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Following a five-week trial at Lewes Crown Court, both Martin and Nathan Winter were found to be "grossly negligent" through knowing an unlicensed metal container packed with fireworks could explode if a blaze broke out.

Their firm, now called Alpha Fireworks Ltd, was convicted of two counts of health and safety breaches in connection with the blast, which injured some 20 others, mainly police and fire officers. It was fined £30,000.

The judge, Mr Justice Cooke, told the father and son in the dock: "You had deliberately placed fireworks capable of causing mass explosion knowing that you didn't have the authorisation.

"You deliberately flouted the explosives regulations for profits, no doubt familiarity bred contempt."

He described Martin Winter as "reckless" in the handling and storage of fireworks and failing to give full and accurate information.

The defendants' family in the public gallery sobbed as they were sentenced.

Both men will be sentenced at 2pm today.

It was while Nathan Winter was making preparations at Marlie Farm for a fireworks show in Eastbourne later on December 3, 2006 that the fatal sequence of events was triggered.

He heard a crack as igniters flared up and soon fireworks were exploding and spreading to other areas of the site. East Sussex Fire and Rescue Service was called and the command passed upwards as more senior fire officers arrived.

One firefighter, Michael Sweetman, described how, moments before the blast, he heard a hissing sound followed by a sucking sound emanating from the container.

Then there was a loud bang, followed by a massive fireball which shot out about 10ft. This was followed by a huge explosion as the container exploded like a bomb and rocked the site, sending fragments flying everywhere.

Dramatic video footage captured by Mr Wembridge was shown to jurors of fireworks exploding and flames engulfing different parts of the site before suddenly going dead.

One fire officer described the scene as like the "Battle of the Somme" as the site was flattened and covered with debris from the massive explosion.

The firework-packed metal container which exploded was unlicensed for storage by Festival Fireworks. And jurors were told it was obvious to both Nathan and Martin Winter of the potential for a huge blast to have occurred if a blaze broke out.

The jury reached their verdicts on Nathan Winter after 19 hours and 46 minutes of deliberation.

Standing beside his father in the dock, he showed no emotion as the verdicts were read out by the jury foreman.

Both men were said to have been familiar with the different hazard classifications given to fireworks, and were aware of the particular danger posed by storing fireworks in a metal container.

Prosecutors also said the Winters, who live on Marlie Farm, were familiar with the licence and its obligations about how and what could be stored and how fireworks should be handled.

But nevertheless, it was said, the container, which had been on the site for years and until recently said to have been used to house chicken feed, was packed with fireworks, including those capable of causing a mass explosion.

Jurors were told the Winters "had a duty of care" to take reasonable care when they went about being involved in the storage and handling of fireworks.

Prosecutor Richard Matthews made the analogy of the car driver who owes a duty of care to everyone, including pedestrians and other motorists, to drive reasonably.

Questions were asked, however, about the role of the fire service after they received the first call to Marlie Farm at 1.45pm, and whether procedures had been correctly followed.

Defence counsel Mukul Chawla QC said if the firefighters had followed their own processes, neither Mr Wembridge nor Mr Wicker would have been near the container when it exploded.

He said that "for whatever reason" it appeared the firefighters had not been provided with any training or had their attention brought to the Health and Safety Executive's Manufacture and Storage of Explosives Regulations.

The regulations state that if a fire is established and involves explosives or threatens to spread them, to evacuate to a distance of 600m, Mr Chawla said. But he added that this was not followed.

During his evidence, Nathan Winter, who was aged 22 at the time, said he stressed the importance for firefighters to keep flames away from the container.

It was suggested that because of his age, the advice was ignored. He told the jury he felt one officer believed he was not in a position to tell him how to do his job.

As Nathan Winter pressed the issue on the site, he was arrested and led off by police before being released. He told a police officer that fire officers were "not listening to him and they needed to pull out".

Martin Winter, who declined to give evidence at trial, said in interview he tried to tell the firemen what could happen and how the fire could be fought, but said: "They wouldn't allow us in there.

Health and safety, so it just took hold."

At one point, the court was told, that Mr Wembridge was ordered to withdraw from the scene after being seen with his camera, with one firefighter telling him: "Brian, get yourself out of here now."

Mr Matthews said that it was not the prosecution's case that the Winters intended to harm or kill the two firemen, but that anyone involved in storing or handling fireworks had a duty to take proper care.

Their failure to take reasonable care amounted to "not simply negligence but gross negligence". The potential for a mass explosion was "obvious and it was well known" to them.

Footage was shown to the jury of controlled explosions in a metal container similar to the one that exploded at Marlie Farm, under what was called the Chaf project.

The project was an EU-funded scheme to investigate and learn lessons from the hazards posed by fireworks and mass explosion in a metal container.

The company was fined 10 years ago after admitting storing fireworks at nearby Upper Lodge Farm, which was not covered by its licence.

Enforcement action was taken under its previous name, Sussex Fireworks and Displays Ltd, and it was fined £1,000 plus costs in December 1999 for contravening Section 5 of the Explosives Act 1875.

Following the convictions Neil Morton, the Health and Safety Executive's chief inspector of explosives, said: "This case is a stark reminder of the terrible consequences of not following the correct procedures when handling hazardous material.

"If Alpha Fireworks had handled and stored the fireworks correctly, the fire and subsequent explosion would not have happened.

"Companies that work in the high hazard industries must remember that regulations and standards are there to protect workers and the public.

"Complying with the law and following the well-established principles of good work practices when using and storing explosives can prevent people being killed or hurt."