It is when Paddy Hill reenacts the violence of the police – who tried to force him to confess to one of the most deadly terrorist bombings in Britain – that he is most alarming.

Looming over me in his cluttered, pet-filled Scottish farmhouse, Hill thrusts his face into mine and grips my knees in a vice-like grip. Contorting his face in simulated fury, he shrieks the obscenities that were hurled at him during the police interrogation, the day after 21 people were killed and 162 others injured in the 1974 Birmingham bombings.

"They jammed a pistol in my mouth and smashed it around, breaking my teeth so badly it was agony to even have a sip of water until I finally saw a dentist, two weeks later. They told me they knew I was innocent but that they didn't care: they had been told to get a conviction and that if I didn't admit to the bombing, they would shoot me in the mouth. They slowly counted to three, then pulled the trigger. They did that three times. Each time, I thought I was going to die," says Hill, pulling up his lip to show his toothless upper gum before rolling down his trouser leg to reveal scars and cigarette burns he says were meted out to him later by the same policemen.

It is tempting to assume that, since his release almost 20 years ago, Hill, now aged 64, must have slowly recovered not only from the inquisition – which left him so battered that his two-year-old son needed medication to recover from the shock of seeing him afterwards – but also from the hell of the 16 years of wrongful imprisonment that followed.

The six innocent men were, after all, later awarded compensation ranging from £840,000 to £1.2m. Surely they were also given counselling? Surely they were not just left to cope with their fury, trauma and wasted decades?

But they were. In the 20 years that followed his release on 14 March 1991, Hill has had to fight for help; a battle he has, until now, failed to win.

Quickly spending his compensation money trying to buy back the love of the family he had lost during his years inside and on helping other innocent prisoners still behind bars, he could not afford to pay for medical expertise for himself. Strugging to function in an unrecognisable world, unable to comprehend the depths of his own disturbance, Hill was reduced to ricocheting around the NHS.

Over the years, without funding or guidance, Hill has managed to convince some of the country's best psychiatrists to see him on a pro bono basis. They did their best but, one after another, have been forced to admit they do not have the expertise to help anyone so acutely traumatised. Hill's various GPs offered him drugs, which he refused. "It's not a depressive thing. It's mental. It's my head I need sorting out. I don't need filling full of pills," he says. The charities he approached were forced to turn him away, because their funding only allows them to help with the rehabilitation and resettlement of guilty prisoners.

"There was no lack of money for falsely imprisoning us, torturing us and putting us through a kangaroo court," Hill says. "But when we came out, there was a sudden shortage of memory and of money. The victims of the Dunblane shooting or the Paddington rail crash, for example, they got counselling immediately, as they should have done. But we were victims of the state: it was the state that took us hostage and traumatised us and now they don't want to recognise that in any shape or form. In the end you give up fighting for help."

Over the last 10 years Hill has been visibly shrinking. Twenty years ago, he was a strong, stocky man weighing more than 12 stone. When he appeared on the steps of the court of appeal on 21 November 1991, a free man, he appeared resilient and determined to forge a future.

The psychological stress of the intervening years has, however, played a cruel physical game. Now he is barely nine stone, his hands shake and his face is wizened. He looks broken. But woe betide anyone who mistakes his physical frailty for weakness or defeat: with every year that passes, the tension inside Hill increases. As he rolls cigarette after cigarette in his sitting room, he is as taut as a wire, veins throb in his neck and, even when he speaks gently, he boils with barely repressed fury.

"Every day, all day, all I think about is getting a gun and shooting police. But I'm not evil: I'm traumatised and I desperately need help," he says. "I'm coming apart at the seams. I can't live in this world because after 16 years in jail, I'm not equipped to deal with it any more. The intervening years have made it worse. I'm like a hand-grenade with a loose pin, just waiting to explode.

"Prison kills you emotionally. It's a dark, deep, evil, brutal world filled with anger, violence, jealousy, paranoia. You become brutalised – it's like being in a war zone," he adds. "Prisons are human dustbins. They're full of people who would kill you at the drop of a hat. For 24 hours a day, every day, you're at risk of being stabbed, slashed or having boiling water thrown over you. After a while, it doesn't mean anything if you see that sort of thing happening to other prisoners. You don't feel a thing. It becomes normal to see someone with a big blade sticking into them or be sitting watching TV and have people burst in and throw boiling water with sugar in over someone sitting near you. You don't blink. It doesn't mean anything to you. I became dehumanised and I still am dehumanised."

Shortly after Hill's release, Dr Adrian Grounds, a forensic psychiatrist with expertise in the psychological consequences of wrongful imprisonment, agreed to see him. Without funding from the NHS, the appointment was a personal favour. The diagnosis, however, was no less brutal for the kindness with which it had been offered. "He said the damage done to me was irrevocable but that I needed at least 10 years of intense counselling, starting immediately, otherwise my condition would get worse as time went on," says Hill.

But nothing happened. No help was offered and Hill didn't know where else to turn. Instead, he closed in on himself.

"My flashpoint is very low: anger comes over me in waves and, over the years, it has got worse and worse," he admits in sad mortification. "I'm too paranoid to socialise. I don't sleep and I don't eat. If I had a choice, I wouldn't live with me. Suddenly the shutters come down and I'm reliving it all again; all the horror, all the torment. When I come to, it's hours later and everyone's gone, and I'm still sitting there, staring at the walls with tears pouring down my face.

"Prison killed me: I am dead. I have had to explain to my kids that I feel nothing for them. I have had to tell them I would rather spend my time with strangers than with them, because you expect to feel nothing for strangers. I hardly ever see my kids now. I can't handle relationships."

Ten years ago, Hill married Tara, an artist he met at a fundraising event for the Miscarriages of Justice Organisation. A warm, practical woman, she tolerates Hill's disappearances – last Christmas, he disappeared for three days – his moods and his furies because, she says, she simply feels so terribly sorry for him. "He's such a tortured man: my heart just goes out to him. Yes, he's difficult to live with but it's not his fault; it's because of what's been done to him by the state. He's such a gentle man. He's just such a sad man. So damaged."

Last month, Hill was suddenly told that he had been given funding by his local NHS Ayrshire & Arran health trust for two months' care at London's Capio Nightingale Hospital with the one man in the country who might be able to understand him: Professor Gordon Turnbull, the only consultant psychiatrist in Britain sufficiently specialised in the psychological after-effects of trauma to help Hill. Turnbull, who counselled the Beirut hostages Terry Waite and John McCarthy, and survivors from the Lockerbie bombing and the Gulf wars, says Hill is one of the most traumatised people he has ever come across.

"Being the victim of a miscarriage of justice in your own country is very much more traumatic than being a conventional hostage, who has been held against his wishes in a foreign country by people who have a different belief system," he says.

"It's totally shocking that there is no method of helping these victims re-emerge into society. The state makes less provision for their release than those who have been rightfully imprisoned. The state has an obligation to rehabilitate these victims."

Gareth Peirce, the solicitor who represented the Guildford Four and the Birmingham Six defendants, has spent years fighting to get help for Hill and other victims of miscarriages of justice. "When the men came out, it was around the time of the Beirut hostages and the papers were full of how they were going to start a programme of support – and not just for them, but for their families too," she said. "It seems to me that the analogy was so close. These men here had come out of a trauma of immense proportion where they were held hostage in their own country. They emerged surviviors from that extreme trauma but without knowing the extremity of it and its effects."

Pierce says the failure of the government to provide appropriate treatment for Hill is a "national disgrace". "Now he's been offered some help but will it even scratch the surface?" she asks. "They have actually been denied the best expertise for 20 years. They were thrown out on to the pavement and no expertise was made available to them. They were left, stumbling around for help. They didn't know where to go and the state didn't offer it."

Hill had to set up the Miscarriages of Justice Organisation – Mojo – to help other former prisoners, released by the court of appeal after their sentences were quashed. "It's the survivor providing help for other survivors," says Pierce. "It's incredibly impressive but horrible and tragic too. Over the years, the government has repeatedly promised these survivors to set up some sort of refuge for them and every promise has not been fulfilled."

Back in his farmhouse, with the prospect of finally getting the help he has fought for, Hill admits he is now wrestling a new enemy: fear. "I'm scared," he says "I'm scared of the anger counselling will unleash. What if there's too much to put back in the box? What if there's too much to contain?

"I'm not expecting miracles though: I'd settle for just stopping the nightmares and the flashbacks. I'd settle to just be able to stop me crying. I just want to know what it's like to feel happy again. I want to feel normal."