In the plush surroundings of a central London hotel foyer, a motherly middle-aged woman is sitting chatting with an attentive young man. Meeting Nuala Gardner and her son, it’s hard to believe that 15 years ago Dale, now 19 and studying for childcare qualifications, was diagnosed with severe autism.

The pair have come to London from their Scottish home to publicise the story of Dale’s journey through autism. A Friend Like Henry, written by Nuala, spares few details of the behaviour linked to the condition – tantrums, kicking, stripping off clothes, obsessions. In her matter-of-fact way she even describes how she, a brisk nononsense nurse, prepared for suicide with a fatal dose of painkill-ers when Dale’s early years threatened to overwhelm the family.

The turning point came with the arrival of a golden retriever puppy, called Henry after Dale’s favourite train in one of his childhood obsessions, Thomas the Tank Engine. Thereafter the five-year-old, previously imprisoned by autism, began to move towards what Nuala describes as “the normal quality of life” he enjoys today.

“Before Henry came Dale had a vocabulary of 30 words,” explains Nuala. But the first morning Dale played with the puppy his astonished parents heard a stream of words burst forth from their son. “We’d never heard our son play so verbally or joyfully before,” Nuala writes.

It may seem like some overblown animal enchantment but, the way Nuala describes it, progress was practical and painstaking. Henry gave Dale confidence, soothing the terrible anxiety autistic children endure and the rituals of grooming, feeding and toilet training the puppy helped the little boy understand how such rules could apply to him too.

When, in the middle of a tantrum, his father James assumed the dog’s voice, saying: “Dale, this is Henry speaking, I hate it when you cry . . . please stop this,” there was another breakthrough. Dale immediately calmed down and from then on whenever their parents wanted to get a message through to their overwrought son, pretending they were Henry did the trick.

The “goldie” even enabled Dale, who like many autistic children, had difficulty expressing emotions, to state his love for his parents: uttering words Nuala feared she would never hear from her son, “Mum, I love you.”

Why did Henry succeed when so much else had failed? Dale, who has written the last chapter of the book, says: “Henry was just really gentle, friendly and sociable . . . he had a wise look and I always trusted him.” Autistic children find it almost impossible to understand facial expressions but the dog made it easier. “I could understand his feelings from looking at his eyes and face,” says Dale. “Henry’s face only had slight changes with his expressions so I understood them.”

Once they realised how happy Henry had made Dale his parents used the puppy to the full. “Dale was unaware how we tapped into the dog,” says Nuala. “He was an educational resource though he was also a pet. Through meeting the dog’s needs Dale was socialising and becoming independent and his confidence and self-esteem went through the roof.”

“Magic” it may have been, as Nuala says, but Henry was also part of a routine by Dale’s parents, grandparents and teachers, who, from the earliest days used one-to-one methods to break into the little boy’s world.

Their aim was to enable him to go to an ordinary school, have friends and hobbies (Dale is now a talented guitarist), pass exams and train for a job. Nuala even introduced him to the collecting card game Pokemon so that he could make friends, and taught him when to say “f*** off” to stop kids bullying him: bullying being the main reason one in five autistic children give up on mainstream education.

As Dale says, Henry speeded up a process that was already happening, albeit slowly. Nuala quantifies it: “Dale was making 25% progress, but when he got Henry there was 50% on top of that.”

Animal therapy is not an answer for all: many autistic children are afraid of dogs. But when it works it can have spectacular results. Dale’s sister Amy, 7, is also autistic, though her condition is less severe. Animals have helped her too – in her case horses, an obsession that started with a ride on a Blackpool donkey. She starts pony club this week, and her mother has even managed to overcome her own phobia of horses.

The family’s story, made into a tele-vision drama starring Keeley Hawes and Sheila Hancock last year, is a heartwarming tale. But there are episodes so bleak they infuriate: such as that before Dale’s diagnosis in which Nuala got the impression professionals suspected she might be suffering from Munchausen’s syndrome, the controversial ailment whereby parents are accused of inventing illnesses in a bid to get attention.

Many parents, she says, are still suffering, struggling to get a diagnosis for their child and after that the correct treatment. What is more, autism appears to be on the increase. “When Dale was diagnosed it was thought to affect one child in 2,000; now they are saying one in 100,” says Nuala.

Sadly, Henry died recently. His successor, “wee Henry”, will be wholly Dale’s responsibility. “I will always want to have a dog in my life,” he says.

“I think when Dale is married I will be going round to take his dogs out for a walk. That is my vision of the future,” says his mother.

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