A KNIFE attacker is behind bars after being arrested four years after the offence by his victim - who had since joined the police!

Mark Rock, aged 26, of no fixed abode, lunged at Mark Deveraux while posing as a cop during an incident in February 2002, cutting his arm.

Rock had caught a couple having sex at the back of the Turks Head pub, High Street, Brierley Hill and demanded £20 from the man.

He claimed to be an off-duty policeman when Mark Deveraux came on to the scene but Mr Deveraux became suspicious after quizzing Rock about police procedure.

During a scuffle, Rock produced a chain and padlock from his pocket and threatened Mr Deveraux, saying: "I am going to smash your face in, do you want to spend the night in Russells Hall?"

He then slashed Mr Deveraux before racing away from the scene.

During last summer, Mr Deveraux, who later became a police officer, was on duty in West Bromwich when he spotted Rock and arrested him.

Wolverhampton Crown Court was told Rock admitted assault causing actual bodily harm and he was put behind bars for nine months.

Judge Michael Dudley ruled the sentence on Rock should run concurrently to a ten-month term he is currently serving.

The judge said: "What you did was quite serious. Had I sentenced you nearer the time the sentence would have been much longer, but a lot of water has passed under the bridge since."