Nothing on that balmy August evening seemed even slightly out of the ordinary. Peggy Hodgson was busily tidying up her terrace house in Enfield, North London, after her four boisterous children had once again left it looking like a pigsty.
Mrs Hodgson's daughters were upstairs getting ready for bed. As usual, 11-year old Janet was playfighting with her elder sister, Margaret.
Then, as the pair rolled around and giggled on the bed, something most peculiar happened: a chest of drawers began sliding slowly across the floor towards them.
The two sisters watched aghast as the chest shuffled across the room as if dragged by a pair of powerful but invisible hands. They were even more afraid when they realised that the piece of oak furniture was about to block their bedroom door - their only means of escape.
Luckily for the children, their mother burst into the room to complain about the noise. She grabbed the chest and shoved it back against the wall. But the invisible force continued. Peggy watched in terror as the chest once again began sliding across the room.
This time, the piece of furniture moved far quicker and Peggy could do nothing to stop it. She tried again to shove the chest back against the wall but failed. And this time she could feel an inhumanly strong force in the room.
Confused and terrified at what she was witnessing, Peggy gathered up her children and fled the bedroom in panic. And thus began one of the strangest cases of alleged haunting ever recorded in Britain.
Over the following months the so-called "Enfield Poltergeist" turned the lives of the Hodgson family upside down. Toys, plates, cutlery, books and pictures would all inexplicably fly across the room. Objects would miraculously appear and disappear before the eyes of terrified onlookers.
Such encounters may sound utterly absurd. But what makes the Enfield case so remarkable is that the events were exhaustively investigated by respected academic researchers and - more pertinently - were witnessed by more than 30 independent witnesses, including police officers.
Although the haunting happened 30 years ago, Janet and Margaret have not spoken publicly about it since childhood. They are still wary about discussing the incident in depth, as their lives have moved on.
But in a documentary to be shown tomorrow they will break their silence for the first time - and what they reveal sheds new light on one of the most remarkable paranormal incidents ever to take place in Britain.
"I felt used by a force that nobody understands," says Janet. "I really don't like to think about it too much."
"I'm not sure the poltergeist was truly 'evil'. It was almost as if it wanted to be part of our family. It didn't want to hurt us. It had died there and wanted to be at rest. The only way it could communicate was through me and my sister."
Hard-bitten sceptics, of course, scoff at such suggestions and claim that poltergeist stories are simply the result of hoaxing and trickery.
They point out that pre-teen girls are hardly reliable witnesses - and, crucially, Janet and her sister have admitted to playing tricks on some of those who were sent to investigate their haunting.
Spooked: Janet says a poltergeist made her fly across the room
This has made it easy for rationalists to dismiss the whole tale as hokum.
And yet a close examination of the story reveals the truth is rather more complex, intriguing and perplexing.
Above all, those who witnessed the events at Enfield were left in no doubt that they were involved in a genuine case of haunting, and while their testimony may seem far-fetched, it is equally improbable to suggest that so many adult witnesses could have been hoaxed.
Key among the independent witnesses were the police officers, who were called to the Hodgsons' home, soon after the poltergeist first made its presence felt in August 1977.
They took statements and noted the family's sincere terror, but in the absence of any hard evidence were sceptical about what might have taken place.
It was only as the officers were preparing to leave that they were forced to take the case more seriously: quite suddenly, a sitting room chair levitated off the carpet before their eyes and started moving slowly across the room.
"It came off the floor nearly half an inch," recalls WPC Carolyn Heeps, one of the Metropolitan Police officers sent to investigate the haunting.
"I saw it slide off to the right about four feet before it came to rest. I checked to see if it could have slid along the floor by itself.
"I even placed a marble on the floor to see whether it would roll in the same direction as the chair. It didn't.
"I checked for wires under the cushions and chairs and I could not see any. I couldn't find any explanation at all."
But, of course, no actual crime had been committed, so the police were unable to assist further.
Desperate for an explanation about what could be taking place in their home, the family turned to the Society for Psychical Research, a respected scientific body that examines cases of alleged haunting from an academic perspective.
It sent two investigators, Guy Lyon Playfair and Maurice Grosse, to examine the evidence. And to avert any claims of trickery, the society drafted in an independent barrister, Mary Rose Barrington, to doublecheck all of their work.
This would ensure that there could be no credible claims that the pair were being anything other than meticulous, honest and impartial in their investigations.
Sure enough, over the following 14 months they spent on the case, the two researchers catalogued a range of inexplicable phenomena.
Boxes flew across rooms, ornaments floated in mid-air, books mysteriously appeared and disappeared. Strange knocking sounds were heard inside walls.
It was all very peculiar. But there was worse to come. One morning when Guy Playfair was working at the house, he heard a "tremendous vibrating noise".
"I really thought someone was drilling a great big hole in the wall of the house," he says. "I tore into the bedroom and there was quite a commotion. The whole fireplace had been ripped out.
"It was one of those old Victorian cast-iron fires that must have weighed at least 60lb. It was so heavy even I couldn't pick it up.
"The children couldn't have possibly ripped it out of the wall. It just wasn't possible. We caught the incident on audio tape, including the fireplace being ripped out of the wall."
Events were soon to take an even more disturbing course. Late one evening, when the children were asleep in their rooms and Maurice Grosse was downstairs compiling his day's findings, he was disturbed by the sound of Janet screaming.
Maurice ran to the foot of the stairs only to see the 12-year-old apparently being dragged through her bedroom door by an unseen force. Janet was then hauled down the stairs and dumped unceremoniously at Maurice's feet.
This incident was also caught on tape and was just the first of several incidents in which the poltergeist apparently picked up Janet and tried to carry her off.
Soon afterwards Janet was even seen floating in mid-air - and this time there were two independent witnesses. A lollipop lady and a passing baker both glanced up at the house and through a top-floor window saw Janet apparently hovering above her bed.
As Janet herself recalls it: "The lady saw me spinning around and banging against the window. I thought I might actually break the window and go through it.
"A lot of children fantasise about flying, but it wasn't like that. When you're levitated with force and you don't know where you're going to land it's very frightening. I still don't know how it happened."
And that wasn't all. Apparently in the grip of some disturbed force, Janet began swearing and hurling insults at those in the room in a disembodied voice quite unlike her own.
The investigators began interrogating "the spirit" - and the answers they got were decidedly sinister.
The poltergeist identified itself as a man named Bill, who explained that "I had a haemorrhage and then I fell asleep and I died in a chair in the corner downstairs."
What could this mean? Astonishingly, subsequent research showed that long before the Hodgsons had moved in to the house, an old man called Bill Wilkins had indeed lived there. And he had died of a brain haemorrhage while sitting in a living room chair.
It's certainly an intriguing tale. But is it really a proven case of a poltergeist?
If you dig deep beneath the surface, doubts soon begin to emerge. On several occasions the girls at the centre of the case were caught playing hoaxes on their investigators.
In one instance, they were caught hiding Guy's tape recorder. They planned to pretend that the poltergeist had whisked it away.
Unfortunately for the girls, the recorder was running and caught their plotting on tape. "They weren't very good at playing tricks," recalls Guy. "We always caught them out. What do you expect children to do?
"I would have been more worried if they hadn't played around from time to time. It means they were behaving like normal kids."
Asked about such pranks today, Janet explains that she and her sister did indeed play practical jokes - because they were so fed up of being tested all the time. They had become like animals trapped in a zoo, constantly being asked to perform tricks for gawping onlookers.
People would turn up expecting inexplicable things to happen, and when nothing happened, the girls decided to play the occasional prank.
But, crucially, Janet estimates that only about one or two per cent of the many hundreds of separate paranormal phenomena that took place in the house were faked by her and Margaret - and these were minor things like balancing a chair on top of a door and pretending that the poltergeist had done it.
Besides, in many cases, it would have been physically impossible for the two young girls to have faked the evidence. How does a 12-year old girl rip out a fireplace, or make a chair levitate in front of police officers?
The barrister, Mary Rose Barrington, who reviewed the case on behalf of the Society for Psychical Research, is in no doubt that the investigators did a thorough and honest job.
She re-interviewed and cross-examined many of the witnesses and double-checked the evidence. Nothing she found suggested a wider conspiracy.
Equally, the 30 or so other witnesses who saw the hauntings - including police officers, journalists and passers-by - all seem convinced by what they saw.
Nevertheless, some experts remain unconvinced. Professor Chris French, a psychologist at London University, is in no doubt that the girls were mischievous and enterprising: "Children can be very ingenious. I don't buy the idea that kids can't outwit intelligent investigators.
Incredible though the events of Enfield were, they are far from unique. Professor David Fontana, a Fellow of the British Psychological Society, has investigated similar hauntings.
He says: "From my own studies I know of accounts of poltergeists pulling people's hair, causing objects to disappear before returning them in the most unlikely places, starting small fires, throwing water about, upsetting furniture, scribbling on walls, breaking objects and generally discomforting the hapless owners of the property they choose to haunt."
Perhaps the last word should go to Janet herself. Now aged 41, and eager to keep details of her present-day life private for fear of attracting ridicule, she is adamant that what she experienced was a genuine paranormal entity.
"I know from my own experience that it was real," she says. "It lived off me, off my energy. Call me mad or a prankster if you like. Those events did happen. The poltergeist was with me - and I feel in a sense that he always will be."