The pair of underage girls groomed for sex by a 50-year-old paedophile, Peter Mitchell, could not attend his sentencing at Derby crown court on Tuesday for one simple reason: they did not exist.

The satisfaction of justice was instead enjoyed by a middle-aged married couple who have become the latest in a wave of vigilante “hunters” who are so frustrated at police inaction to stop online grooming of children that they are taking the law into their own hands.

Mitchell received a three-year community order and was placed on the sex offender register after pleading guilty to grooming “Laura” and “Roxy”, two fictitious 14-year-olds he met on Facebook, and possession of images of children. The teenagers – complete with phoney enthusiasms for the singer Ed Sheeran and labrador puppies – were invented by Brendan Collis, from Derby, and his family under the name Online Predator Investigation Team (OPIT).

The Collis family watched from the public gallery as Judge John Burgess told Mitchell that there was public revulsion at his “dreadful” crimes. The groundsman, from Blidworth, Nottinghamshire, worked in schools and sent the fictitious girls pictures of his penis while sitting on his lawnmower.

Despite knowing OPIT’s girls were underage, Mitchell told them about his sexual urges and his desire to get them drunk, and arranged to meet them.

Brendan Collis explains his group’s tactics Guardian

It was the latest of several successful prosecutions following stings by vigilante groups who say they are filling a vacuum left by inadequate policing of the internet. The groups have, however, faced criticism for putting lives and evidence at risk.

The prosecution came as one of the country’s leading sex crime experts, Jim Gamble, former director of the national Child Exploitation and Online Protection agency, called for vigilantes to be brought in from the cold to form “a volunteer army” of 1,000 “digital detectives” managed by police.

Gamble attacked the “confused, muddled and incompetent approach by government” to patrolling the internet and said each force could train 20 volunteers to work in police stations on police computers equipped to gather evidence for prosecutions. Such a scheme would cost £1.5m, Gamble said.

“The vigilantes won’t go away until we give people a viable alternative,” he said. “I understand the type of anger that fuels many of the vigilantes who go online but they are not the answer. They do, however, prove that you don’t need to be a police officer to catch these people.” Growing public support for anti-paedophile vigilantes was revealed last month when the Nuneaton-based Stinson Hunter team crowdsourced more than £32,000 to support its controversial video investigations, which it claims have led to 15 convictions.

Following the Mitchell case, Toby Fawcett-Greaves, who leads Derbyshire police’s public protection department, said: “We always welcome and encourage any information that is passed to us that will help to protect the safety of children.

“There is always a risk that any information passed to us by individual vigilante groups may not be accepted by the courts. Any intelligence received will be investigated and the police will always act fairly, impartially and comply with legislation.”

Collis said he was partly inspired to try to catch paedophiles because of his experience of paedophiles being introduced into his own family when he was younger and the suicide of a family member following child sex abuse.

“We need to send a message out that people who try and attempt this are punished,” he said after the sentencing. “This doesn’t just happen in Facebook chatrooms; it is happening across the internet.”

To trap Mitchell in August 2013, OPIT set up seven false Facebook profiles of girls aged 12 to 14. “Laura Chambers” was pictured cuddling a puppy and within minutes of connecting, Mitchell told her he had been “pervin at ya pic”.

In hundreds of messages, seen by the Guardian, he said he planned to buy her vodka and a camera so she could send him intimate pictures. He then sent her a picture of his penis and over the following days kept up a torrent of lewd suggestions. Mitchell proposed meeting but said: “I don’t want to bump into ya mum”.

He said he wanted to have sex with her and they settled on meeting in Alvaston Park, Derby, and that he would bring vodka, saying: “After a bottle ya see how loose ya jeans go”.

While he was grooming Laura, he also started grooming Roxy Hollis, another OPIT creation who described herself as a fan of Britain’s Got Talent.

She told him she was 14 and he said he was 48. Later that evening he sent her a picture of his penis with the message: “horny hehe”.

“Roxy” replied that he was being “cheeky” but he continued to talk explicitly to her and said: “Soo ya gonna let this old man come see ya 1 day”.

She asked when and he replied: “Haha you get me locked up”. Mitchell and Roxy planned to meet in a Tesco car park in order, Mitchell thought, to have sex.

Then the day before their meeting he suddenly told Roxy he had just seen Laura and “got ma pic taken cos it is a set up”. OPIT had confronted him.

“We followed him into the park,” Collis said. “We filmed him and he sped away,” apparently almost knocking over one of the OPIT team.

Mitchell sent a message to Roxy: “don’t know what to do hang myself prob”. It was only 10.10am but he said he was going to buy vodka and tablets.

“We had to pull the stops out to make sure he didn’t,” said Collis. “My wife started chatting to him [using the Roxy cover identity] and tried to keep calm and then he said OK, let’s meet.”

“Roxy” made up a story about how her mum and dad were still in bed and that she would sneak out so they could meet at a Tesco, several miles away in Alfreton, Derbyshire. He decided to go.

“At that point we knew he was not going to kill himself,” Collis said. “He wanted to have his way with someone.”

They confronted and filmed Mitchell again, gathered the digital evidence on to CDs, including 3,500 messages sent over one week, and took them to Derbyshire police, which voiced mixed feelings about vigilante groups on Tuesday. A force spokeswoman added: “We would advise to leave any type of investigation like this for the police to deal with. Any information is gratefully received but thereinafter the police will conduct inquiries.”

The court heard Mitchell had made several changes to his lifestyle since the sting, including replacing the phone that allowed him constant access to Facebook with a simpler model and changing jobs so he has no contact with children.

Asked outside Derby crown court whether he felt ashamed of what he had done, he replied “yes” and declined to comment further.