A taxi driver heard an “argument” moments before two men started “grappling in the street”, a court has heard.

Jason Stewart was inside his cab when Andrew Boag and Stephen Lowe fell onto his passenger seat while they were “struggling against each other”, he told St Albans Crown Court today.

Mr Boag, 35, was killed in an incident that took place in Garsmouth Way, Watford, on Friday, July 25, 2008. Lowe, who is known as Frosty, has pleaded not guilty to murder.

The court heard that Mr Stewart, who worked for Eagle taxis, would act as a driver for Mr Boag once or twice a week when he completed drug deals.

On July 25, Mr Stewart was at home when Mr Boag rang his mobile phone, the jury heard.

The witness picked up Mr Boag from the Badger pub, in The Gossamers, at around 8pm and headed along the A41 towards Harrow, the court heard.

Then, after Mr Boag received a phone call, the driver was asked to turn his blue Mercedes company car around and head towards Garsmouth Way, he told the jury.

Mr Stewart said once he had parked the car, Mr Boag went to complete a “drug deal” with a young woman, and was away for five minutes.

Then, as he read a magazine while still sitting in the driver's seat, he looked up and saw two men “grappling”, the court heard.

He said: “Andy fell into the car backwards, with his back against my side. Frosty wasn't in the car fully at the time. With the force of Andy struggling, it forced me to go side on to the driver's window.

“I panicked. I'm not into violence at all. I grabbed the keys, I don't know why, and grabbed the door. I had to put my hands on the floor outside to get out of the car.”

Mr Stewart, who described the incident as “traumatic”, told the court the young girl at the scene was “hysterical and screaming”.

He said: “She was saying: 'Leave him, get off him. He's not breathing, you've killed him.' She said Frosty.

“She shouted if I had a tool to get Frosty off Andy but I don't carry tools in the car.”

Questioned by prosecutor Anne Evans, Mr Stewart said he knew Mr Boag carried weapons in his car – one a brass knuckle-duster and a six-inch knife contained in a leather holster.

He added, however, that he had not seen Lowe with an “implement”.

Under cross-examination from Ann Cotcher, QC, representing Lowe, Mr Stewart said he had known “Frosty” since they were teenagers, and considered him “a friend”.

He told the jury: “The last thing I expected from him was to do that. I didn't think he was that sort of person.”

The witness said Mr Boag had told him he visited Lowe's flat two weeks earlier with a gun, but that “Frosty” wouldn't come out. Carrying a gun was a “step too far”, he told the court.

Ms Cotcher asked the witness: “She [the woman at the scene] asked you where's his chiv – a knife. Where's his chiv? She was meaning Andrew Boag's knife.”

Mr Stewart replied: “I said I didn't know. I said I didn't have a tool.”

Ms Cotcher then asked the witness if he heard Lowe tell him: “Tell your man that was a boyslap, not a manslap.”

He said: “I didn't hear him say that.”

The court also heard from a second witness this morning, who saw a “fracas” from his flat window at around 10.10pm on the night in question.

The court heard there was an “awful lot of shouting between various people”, in Garsmouth Way.

The witness said: “I heard a young lady shout: 'That's enough, stop it Frosty, you will kill him'.”

Stephen Lowe, of Breakspeare Close, Watford, has pleaded not guilty to murder. He has also pleaded not guilty to an alternative count of manslaughter.

The trial continues.