When diners at a McDonald’s saw police officers leading away a customer, they would not have known the man who had been drinking coffee was a depraved paedophile.

Former submariner Geoffrey Grimster-Shepherd, 58, had been waiting for a nine-year-old girl after arranging through her "stepmother" to meet and abuse her.

In fact, the whole operation was a police sting and now the former prison officer and naval chief petty officer, who had travelled 260 miles to the Gillingham McDonald’s, has been jailed for seven years and eight months.

The married father-of-three, from Torpoint, Cornwall, had online chats with a woman in which he detailed what he wanted to do with her stepdaughter.

His depraved ideas included her dressing in her school uniform while they performed sex acts on each other.

He also wanted the woman and the girl to have a threesome.

Police later found almost 3,000 images and movie clips of children being subjected to horrific abuse on his computer and on memory sticks, with some victims younger than two.

"You clearly and graphically detailed what you wanted to do" - Judge Julian Smith

Maidstone Crown Court heard Grimster-Shepherd also engaged in online chats with 12 other people who offered their children for abuse, but there was no evidence he ever met up with anyone else.

Grimster-Shepherd, who worked as a support worker for adults with learning difficulties, admitted two offences of arranging or facilitating the commission of a child sex offence.

He also admitted 10 counts of possessing indecent photographs of children.

Judge Julian Smith said he represented a danger to the public and imposed an extended sentence of seven years and eight months.

The court heard his wife and children were standing by him.

Judge Smith said he had no doubt Grimster-Shepherd would have carried out the acts.

Two further offences of possessing indecent images of children were asked to be taken into consideration during his sentencing.

Judge Smith added: “You clearly and graphically detailed what you wanted to do and were excited about what you intended.”

The NSPCC said his behaviour highlighted the dangers youngsters face using the internet.

A spokesman said: “Grimster-Shepherd’s behaviour has highlighted the dangers youngsters face online.

“This kind of predatory activity can ruin childhoods and the devastating effects can last into adulthood."

For tips on keeping children safe online, visit the NSPCC's Net Aware website or the Internet Matters website.