A former Dartford deputy headmaster and running coach who sexually abused a young boy 30 years ago after telling him he was doing research into puberty was jailed for eight months today.

Brian Fagg's guilty secret remained hidden until his victim, having been told Fagg had been spotted with a boy in his car, decided to go to the police.

Maidstone Crown Court heard the 70-year-old, who taught biology at Dartford West Boys' School, touched the boy indecently.

The one-off incident occurred at Fagg's home.

Prosecutor Ian Foinette said that during the abuse the doorbell rang. Fagg then told the youngster of the sex act he would have performed had he not been "saved by the bell."

Jailing the pensioner, who also coached athletes at Dartford Harriers, Judge Philip Statman said Fagg had "used" his victim for his own sexual gratification.

He added that Fagg had little insight into his offending but his guilty plea had spared his victim from the ordeal of giving evidence.

Fagg, of Hallford Way, Dartford, admitted indecent assault on a male person in January 1979.

Fagg was arrested in October last year and initially denied the allegation. However, he made a full confession when he was reinterviewed four months later.

As part of his sentence Fagg was also disqualified from working with children indefinitely, ordered to sign on the sex offenders' register for 10 years and made subject to a Sexual Offences Prevention Order for five years.