A man accused of sexually assaulting a 16-year-old girl at gunpoint while her boyfriend looked helplessly on was one billion times more likley to have committed the crime than anybody else, a jury has heard.

Salisbury Crown Court was told that a DNA match linked Keith Henderson, 57, formerly of Cooks Close in Fugglestone Red to the crime.

The incident happened on Salisbury’s Town Path in the early hours of October 7, 2001, but despite a large-scale police investigation and appeals on the television programme Crimewatch, no suspect was found until a decade later.

Henderson, who was working as a close constable at the Cathedral Close in 2001, was arrested on October 4 last year and charged with indecently assaulting the girl, causing actual bodily harm to the boy, false imprisonment of them both and possession of an imitation firearm, because the authorities cannot be sure if it was a real gun or not. He has pleaded not guilty to all five offences.

On Monday prosecutor Rachel Robertson described the incident as “horrific and terrifying”.

The jury heard the couple, now in their 20s and who can not be named for legal reasons, were walking home from a night out at about 2am when the 16-year-old boy was hit on the back of the head and temporarily lost consciousness.

The man then grabbed the girl, holding the gun to her head, and threatened to kill her if she made any noise. He forced them both to climb over a gate into a neighbouring field and made her boyfriend lie flat on the ground while he forced the girl to perform a sex act.

Despite being threatened by her attacker, the victim managed to ensure there was DNA evidence on her clothing, which the police later recovered. Although no match was found at the time, Mrs Robertson told the jury a familial link with the offender in this case was found when someone was arrested and cautioned for a minor matter in 2007.

Forensic scientist Jonathan Whitaker told the court a DNA profile taken from Henderson when he was arrested last year was a full profile match to the stain on the girl’s trousers.

Dr Whitaker said the statistics showed it was a billion times more likely the DNA came from Henderson than anyone else.

When police searched Henderson’s home they found a box of bullets from a semi-automatic hand gun.

Henderson told the court he has served 22 years in the army and was awarded seven medals for good conduct, long service and operational tours. He is a man with no previous convictions and said he has no experience of hand guns, having only used a rifle in his army career.

When interviewed by police he gave a no comment response to all police questions. In court he said he did not carry out the attack and cannot explain the DNA evidence.

Henderson said he had found the box of bullets on Salisbury Plain and kept them “because he was a hoarder.”

For trial updates visit salisburyjournal.co.uk