A predatory paedophile who posed as a children's author on a social networking site to infiltrate a group of 14-year-old girls before following them on a school trip has been jailed.

Unemployed Ian David Hunter, 55, of Chiswick, was jailed for two years today after pleading guilty at Inner London Crown Court to the sexual assault of a 14-year-old girl and 16 counts of possessing indecent images of children.

The court heard Hunter had joined a social networking site popular with teenagers and infiltrated the girls' group by posing as a 26-year-old woman calling herself 'Popcorn Puppy' and 'Lawndog' as well as 'JL Seagull' - a children's author from Brighton.

Being part of the group let him see messages between the girls as well as photos of them, their friends and family.

He also posted messages to them, trying to fit in by using abbreviations and slang.

After being accepted in to the circle he owned up to being a man in his 50s.

Most of the girls then rejected Hunter, stating they found his presence on the site inappropriate.

But as not all had rejected him he was still able to view their messages and from this learnt of a school trip to the Tate Modern art gallery in Southwark.

Approaching one of the girls at the gallery he introduced himself, saying he was "not a pervert" but simply wanted friendship while he sexually assaulted her by touching her back.

Fearing Hunter's motives, the girls took photos of him on their mobile phones as well as calling for teachers and security guards.

Hunter was kicked out of the gallery and reported to police before being arrested on a warrant on November 17 last year.

A search of Hunter's room led to the seizure of computers with 21,242 indecent images of children, including many movies.

Police said a high proportion were of a horrific nature.

The investigation also uncovered that Hunter had applied for caretaker positions at two London girls' schools as well as communicating with a jailed registered sex offender.

Prosecutors at the trial in September were granted permission to tell the jury of Hunter's possession of the indecent images to demonstrate that he was not just a lonely man trying to make friends but was in fact a predatory paedophile who in travelling to meet the girls had gone one step further to fulfil his fantasies.

After today's sentencing, Detective Inspector David Gray said: "This is an insidious crime committed by a predatory male."

Two charges of grooming were ordered to lie on file.