Thirteen other defendants - who include two loyalists once charged with trying to murder Haddock himself - were seated under police guard in a fortified dock.
They stand accused in the largest Troubles court case staged in Belfast since the 1980s. It is expected to last at least 11 weeks.
Heightened security was in place around the courthouse, while in the courtroom more than two dozen barristers are appearing before a judge who is hearing the case without a jury.
The case will hinge on the evidence of Stewart and his brother, David, 41, who three years ago walked into a police station and confessed to their role in the killing and agreed to testify against their former associates.
Although the large-scale supergrass cases of the 1980s were discontinued after widespread criticism, the present case has generated protests in hardline loyalist areas but not further afield. A woman with a placard protesting outside the court yesterday said: "We don't do statements".
Supporters say the use of the tactic is unjust and is victimising loyalists, rather than republicans.
The defendants stared at Stewart as he gave evidence flanked by two police officers. He told the court he and his brother had been among more than a dozen UVF members who gathered in a flat during a feud with another loyalist group. He said Haddock had named the victim and declared: "I want him done - shot dead".
The group organised the killing, commandeering a taxi and gathering weapons, gloves and balaclava helmets. The door of English's home was battered down with a sledgehammer of a type which the gang also used in "punishment" beatings.
English was shot several times, his wife saying that at one point one of the men shouted to the others: "Come on back and finish the bastard".
According to Stewart's testimony one of those involved in the attack had earlier "said he wanted it to be all over so he could go home and have a kebab".
In court, Haddock appeared relaxed, giving a thumbs-up sign to reporters and chatting to the prison staff around him.
- Independent