A HEROIC policeman has described how he grappled with a manic knife murderer on a motorway slip road.

PC Matthew Mayo, 29, of Old Field Drive, Wouldham, near Rochester, intervened when he came across Francois Christophe as he brutally attacked lover Joseph Ennis on the M25 near Swanley.

PC Mayo, who works for the Metropolitan Police Force, armed himself with a Stanley knife as he jumped from a moving van to try to help the defenceless Gravesend cabbie.

He said: "I saw this taxi had stopped in the middle of the road. As we drove past I noticed the driver's foot hanging out of the door. Then I saw a man in the back who had his arm around the driver who was slumped forward. The man was plunging a knife into his chest."

Thinking he was witnessing a robbery gone wrong, PC Mayo ran towards the car while calling for police back-up.

He said: "I kept shouting at him to stop. Suddenly he looked up and stared at me. Then he plunged the knife into the man's neck."

Christophe then got out of the car but appeared to be about to go back to his victim.

PC Mayo said: "I thought he was going to either carry on stabbing him or pull him out of the car and drive away so I kicked him in the chest. He went flying backwards but within a second he was back up again. He turned his back to me so I jumped on him and grappled him to the ground.

"I eventually managed to tie him up and when the police arrived I went to Mr Ennis but quickly realised there was nothing I could do."

Despite being a police officer for the last seven years, PC Mayo says he has never seen something as disturbing as the murder.

He insists instinct kicked in and he did not have a chance to think about what he was doing.

He said: "People say it was brave but you just do what you think is right at the time.

It could have been a lot worse but given the same situation again I would do exactly the same thing.

"I know there was nothing I could do to save Mr Ennis. Had I not intervened I would always have been wondering if I could have saved him."

Christophe, of Maritime Gate, Northfleet, was sentenced to life imprisonment at Maidstone Crown Court after pleading guilty to murder.