A stray cat became the catalyst that allowed Julia Romp's autistic son, George, to communicate with his mother – and even learn to read and write. But then it disappeared and George withdrew again. Juliet Rix finds out how they finally tracked their pet down

Pets: essential members of the family or a damn nuisance to be avoided at all costs? Opinion is divided, but one thing is certain, pets bring out strong feelings and can have a strong impact on family's lives, especially in the case of Julia Romp and her son George.

Julia was determined never to get a cat – her mother rescued cats and as a child she remembers being displaced from the sofa by her mother's feline friends. "I was the child who went to school covered in fur and smelling of cat pee," she says. Finding herself the single parent of an autistic son only strengthened her resolve. She had enough on her plate.

Then Ben walked into her life, a stray cat who was to have a dramatic effect on her views and her family life. "It was definitely not love at first sight," she explains. When the cat first visited her garden in Isleworth, west London – filthy, sick and bloody – it hissed whenever Julia or George came near. Over a period of weeks Julia's resolve about cat ownership was broken as her son George developed a special relationship with Ben.

George, 10, has autism. Before Ben arrived, he didn't understand people and would kick and bite if other children touched him. He never showed affection, yet would stroke Ben and bury his face in the cat's fur. He didn't make eye contact – but looked straight at Ben, and when Ben looked back he didn't flinch. George didn't smile either and only spoke in a monotone. But he began to develop a high-pitched "cat-talk" voice, in which he spoke to Ben and replied on Ben's behalf.

Julia picked up the cat-talk and copied it. Using Ben as an intermediary, she was able to communicate with George. He started telling imaginative stories of Ben's escapades: "Ben was a DJ in 1973 … Ben went to sea in a storm ...", and she discovered that on the visits and day trips she and George and taken over the years, while he had seemed to take no notice of anything, he had, in fact, been drinking it all in.

Over time, George's relationship with Ben rubbed off on his relationships with people. He became calmer at school and better with the other children. He began to learn to read and write. He told Ben he loved him and to Julia's disbelieving delight, he told his mum he loved her too. "I started to take it all for granted," she says.

And then Ben disappeared.

Julia and George were on a rare holiday when the call came. Julia didn't hesitate: "I must have looked like a mad woman rushing around the airport trying to get a flight home." Arriving home at 4am, she dumped the suitcase by the front door and leaving George with her mother, went out to search for Ben.

George became withdrawn. The monotone returned. There was no cat-talk, no laughter. "My life went on hold," Julia says, "I had to find Ben." She scoured the area, contacted every vet and pet shop, printed leaflets and plastered thousands of posters across the borough. The council threatened to take her to court. Her response was: "Oh, yes, would you? That might get it on TV and someone might recognise Ben." The council didn't pursue it.

Julia fielded hundreds of calls from people claiming to have seen her cat. She sat in parks all night, fell into a river and visited countless houses and gardens. Some calls were genuine possible sightings, she says, others "were lonely people … who just wanted a chat and a cup of tea with me". Most were well-meaning, although one woman started telling Julia how her cat had been murdered and so had her mother, then blocked Julia's route out of the house. "That was frightening," says Julia, "She had mental health problems."

Weeks passed. A month. Two months. George withdrew even more. "I had been told not to show too much sadness in front of George, so when it all got too much I ran to the bathroom and cried into a towel." She became so desperate that she rang a missing persons agency and asked how much it cost to look for someone. When they wanted more information, "I had to tell them it was a cat and they laughed."

Christmas was coming and Julia became worried that all the stress and uncertainty was worse for George than Ben being dead. "George worried about Ben. He'd say, 'It's cold tonight. Ben will be cold.'" Julia decided enough was enough and planned a funeral. She would tell George that Ben had had an accident and the vet had cremated him, and they would bury some ashes in the garden. But when George got home and asked, "What's that hole for?" Julia says, "I couldn't do it. I said I'd been doing some gardening."

Then, four days before Christmas, Julia received a call from Brighton. Was she really going to go all that way for yet another hoax or mistaken identity? But the family in Brighton said they had checked the cat's chip and insisted it was Ben. So Julia drove to the coast.

"The house was decorated with the same decorations we usually have for Christmas," she says, "and the family were lovely but I was trembling." A black and white cat wandered out of a cat bed. It was Ben. Julia scooped him up and collapsed in a heap of fur and tears in the middle of the floor. Then she phoned her mother, who was at home with George.

With Ben and George reunited, life resumed. George's cat voice returned, as did the smiles and the laughter. They still don't know how Ben got to Brighton, but George had his own theories. He described how Ben had gone there to get fish and chips by the sea and to visit Katie Price, who lives nearby – Julia breathed a huge sigh of relief. She unpacked the suitcase and hung the Christmas decorations. "I realised," she says, "that I hadn't been searching for Ben just for George, but for myself as well."

The doorbell rings and George arrives home from school. He looks me straight in the eye (albeit briefly) and says hello. Then he goes in search of Ben. "The thing is," Julia says, "in this relationship, George is in control … He does both sides of the conversation, he is the one who touches, he decides what and when to play."

So would she recommend a pet for other children who have special needs? Julia hesitates. She had tried getting George a budgie and a rabbit before Ben arrived, she says, but neither worked. "It has to be the right animal," she adds. I hardly dare ask the next question: What happens when Ben dies? Julia she has thought about that. She is planning to introduce a kitten to the household soon in the hope that George will attach himself to "Ben's son or daughter".

"When I found Ben," she says, "I promised myself I would stop wishing for things – I was always wishing George had a [resident] dad, that I had a job." But "you only realise what you've got when you lose it: George, Ben and I are a family – and we're complete".

That should be the end of the story, but a few days after Christmas the phone rang again. It was a woman in Devon. Her family was distraught: they had lost their cat on a visit to her mother in Isleworth. Would Julia help them find Numpty? "My whole body went goosepimply. I knew how she felt."

So it all began again: the posters, the leaflets, "bothering the local pet shops". "Everyone thought I was crazy" – everyone except George. After eight weeks, Julia found Numpty healthy and happy living with an elderly lady less than a mile away. "She had found him stray and underfed. She told me she had had a lot of sadness in her life and that this cat had come to her and that she loved him."

"It crossed my mind never to tell anyone I'd found it," says Julia, "but I couldn't do that." So she rang the owner in Devon and told her the story. "The family came up and met the lady … They could see the sadness in her and that there had been a gap in her life … They let her keep the cat."

Success doesn't stay a secret for long and Julia is now seen as the local pet detective. She is currently looking for a tabby called Zamba, which has been missing for three months. She is also working as a volunteer for two charities that rescue animals and help people on low incomes to look after their pets. She has recently been advised to apply for an RSPCA training course, which would help her to get paid animal work. She grins: "I might even end up with a job!"

A Friend Like Ben: The Cat That Came Home for Christmas, by Julia Romp, is published by Harper Collins,£14.99. To order a copy for £11.99 with free UK p&p go to theguardian.com/bookshop or call 0330 333 6846