A gang of youths who took pleasure from inflictiing pain began lengthy jail sentences last night for killing a bar manager during an "orgy" of random violence.

David Morley, a survivor of the Soho nail bombing, was beaten to death by the gang while a 14-year-old girl filmed the attack on her mobile phone - apparently part of the "happy slapping" craze. As the man lay on the ground, she kicked his head "like a football".

The gratuitous violence inflicted on Mr Morley, 37, resulted in at least 40 injuries. Fractured ribs ruptured his spleen, causing him to bleed to death.

Chelsea O'Mahoney, now 16, was sentenced to eight years for Mr Morley's manslaughter and five years for conspiracy to cause grievous bodily harm, to run concurrently. Reece Sargeant, 21, Darren Case, 18, and David Blenman, 17, were each jailed for 12 years and six years respectively for the two offences, also to run concurrently. They were told they would each have to serve at least two-thirds of their terms.

The group launched five separate "happy slapping" attacks on eight victims within the space of an hour, near the South Bank theatre complex in central London on 30 October 2004.

Detectives showed the jury at the Old Bailey graphic CCTV footage of the quartet's final attack of the night, an assault on a man sleeping rough in a doorway near Waterloo Station. O'Mahoney could be seen holding her mobile phone in the air, apparently filming the violence. They repeatedly kicked the victim, Wayne Miller, in the head, before running off "whooping" in delight.

"When you first see the CCTV, you are stunned by it," said Detective Chief Inspector Nick Scola, who led the investigation into the spree of violence. "It takes a little time to get used to what you are seeing."

Sentencing the four, the Common Serjeant of London, Brian Barker, said the gang had filmed their assaults for "amusement", "gratification" and "enhanced status".

"You called this 'happy slapping'; no victim on the receiving end would dignify it with such a deceptive description," he said.

"No one listening to this case could fail to have been affected by your selfishness and blindness to the suffering of others. You sought enjoyment from humiliation and pleasure from the infliction of pain."

The gang mirrored the group in the fictional book and film A Clockwork Orange. In Anthony Burgess's portrayal of teenage violence, codewords were used to signal violence.

Defending O'Mahoney, Anthony Berry QC told the court that she was "the victim of a particularly chaotic and fragmented life".

"Both of her parents were heroin addicts," he said. She had been found wandering the streets of London in the middle of the night, completely unsupervised, when aged just three or four.

The court heard that Case was emotionally immature. Sargeant felt deeply ashamed at what had happened, and had learning difficulties and a speech impediment. Blenman too had behavioural and emotional problems.

Speaking outside court, Mr Morley's adoptive father Geoffrey, 76, a retired engineer, said: "Nothing will bring my son back. At least it will be better for the public at large and perhaps better for them that they [the gang] have a sentence that fits the crime."

But he added: "I did not see any signs of remorse on the defendants' faces during the eight-week trial."

Alastair Whiteside, a friend of Mr Morley who watched helplessly as his companion was being beaten, said the group "were sorry - but they were sorry they got caught". Mr Whiteside now suffers from insomnia and has been constantly woken by nightmares since the attack took place.

Mr Morley, from Chiswick, west London, had been working at the Admiral Duncan pub in Old Compton Street, Soho, when it was nail-bombed on 18 April 1999. Mr Scola said that Mr Morley had since come to symbolise "what is good in human nature".

"For him to lose his life in such a cowardly and pointless attack was a tragedy in every sense," he said.