Police and security guards cleared a public gallery amid angry scenes when a drug dealer who injected his supplier with a fatal dose of heroin was jailed.

Mark Dyson, who denied murder but admitted manslaughter, was yesterday sentenced to four-and-a-half years in prison by Lewes Crown Court.

But family and friends of the dead man, Derek Finch, shouted out in protest and one man tried to climb into the dock to reach Dyson as he was led to the cells.

Police and court security officers cleared the public gallery and one man was arrested for a public order offence.

After the hearing, Mr Finch's brother Mark said he was disgusted with the legal system.

Mr Finch, 41, from Chichester, praised the police for their hard work on the case but was unhappy the murder charge was dropped.

He said: "The sentence was not long enough. My brother's memory has been insulted.

Dyson, 35, of Laycock Close, West Wittering, near Chichester, had denied murdering Mr Finch, whose naked body was found at his flat in Chichester last August.

But on the third day of the trial he admitted manslaughter and the jury formally acquitted him of murder.

At the start of the trial, Christopher Clark, prosecuting, told the jury Mr Finch, who lived in Carleton Road, was a drug dealer who had been deliberately and ruthlessly killed by Dyson, who was one of his team of pushers.

He said Dyson owed money to Finch and had been threatened by him.

Mr Clark said it was known Mr Finch was not a regular user of heroin and when he did take the drug he asked someone else to inject him.

He alleged Dyson injected Mr Finch at least twice with heroin after being invited to his flat.

The jury heard a post-mortem examination revealed three recent injection marks on Mr Finch's right forearm.

Tracey Matthews, 26, who lives in Chichester, told the jury that before she and Dyson went to visit Mr Finch, Dyson said to her: "This could be my chance to get Derek out of the picture."

He then said he was only joking.

The court heard how Mr Finch was a heavy user of cocaine, which had caused damage to a heart muscle and could have contributed to his death.

Camden Pratt, defending, said Dyson had injected Mr Finch at his request because Finch had wanted to come down from cocaine.

He said: "It is my submission that there was no intention by this defendant to do any harm.

"In this case it should be assumed there was a damaged heart muscle and so the dose of heroin, acting with that cocaine-damaged heart, was rather like a minor blow to a person with an eggshell skull, which proves fatal.

In passing sentence, Mr Justice Moses told Dyson: "There has been much evidence about Derek Finch and not all of it favourable but that cannot excuse what you did. No one deserves to die in such circumstances."