It is nearly a year since the London bombings, but for the survivors the smallest things - the smell of burning, laughter, being late - still trigger traumas. But what can help overcome memories of being trapped in trains, surrounded by the dying? Alexander Linklater reports.

Not long after escaping the explosion in his carriage on the Edgware Road tube, Thomas, a 38-year-old computer scientist from Germany, began to notice that he had acquired a puzzling psychological tic. It wasn't the worst of his symptoms, only the most tantalisingly specific. It happened whenever he laughed. Admittedly, in the weeks following the London bombings of July 7 2005, Thomas didn't have much to joke about, but when he did find something funny his laughter would be replaced, an instant later, by an overwhelming rush of sadness. Referred to a counsellor by his London GP, Thomas had been told to write down his thoughts when this occurred. But there was nothing to write. He didn't have any particular thoughts; just a jolting switch of feelings. When it was explained to him that the laughter must be a trigger for a suppressed memory, Thomas asked how he should un-suppress it. The counsellor replied, frankly, "I don't know."

It wasn't until he managed to find his way into mainstream cognitive therapy at an NHS psychotrauma clinic in November that Thomas got his explanation. He was suffering from nightmares and flashbacks, and a partial amnesia seemed to be paralysing him. When he tried to concentrate or work, a kind of pressure would build up, as if he could feel it physically in his brain - "like I'm a missile locked on to a target," he says, "going faster and faster, and I can't get out." So his therapist started to help him relive his experience of July 7 in order to regain his memory and, in the argot of cognitive therapy, to "reprocess" it.

While piecing together events before the bomb went off, Thomas told his therapist how he had been sitting on the Circle line train, reading a book by one of his favourite German authors - Wladimir Kaminer, a Russian émigré who writes about absurd and improbable events in a world gone mad. In his "relive", Thomas recalled being in a good mood and finding the book, called Militärmusik, very funny. It was then that the penny dropped. At the moment the suicide bomber Mohammad Sidique Khan blew himself up a few feet away, killing seven people in the train and injuring 163, Thomas had very likely been laughing.

The idea that the human mind can get locked into an exact replay of a past event can seem overly neat, like detective-fiction psychology. But Thomas's automatic flip from laughter to horror - replicating his responses during the event - is just a very precise example of what many others caught up in the London bombings have since been experiencing in varying ways and to varying degrees. The notion that memory contains triggering and blocking mechanisms is one of the most basic premises of current cognitive psychology. What remains controversial is identifying the stage at which a malfunction of these mechanisms can be given a clear medical diagnosis - Post Traumatic Stress Disorder (PTSD).

Almost every single one of the approximately 4,000 people directly caught up in the London bombings qualifies for what is known as "Criterion A" of a PTSD diagnosis. Which is to say, they have gone through a calamity that involved actual or threatened death, and experienced intense fear, helplessness or horror. The degree to which they can be said to be clinically ill as a result depends on whether they also fulfil criteria B to F, which include flashbacks and nightmares, avoidance, hyper-vigilance, exaggerated emotional reactions and an inability to function socially. If a survivor displays enough of these symptoms, he or she will be eligible for Cognitive Behaviour Therapy (CBT), now considered in Britain to be the gold-standard trauma treatment. If not, then a GP may be able to offer a more general counsellor, or antidepressants. But getting cognitive therapy on the NHS is not easy, and involves filling out a series of questionnaires that will provide a PTSD score - a process that can be at once formulaic and ambiguous. Some people may downplay their symptoms, others may cry out for attention or - as some did with their London bombing questionnaires - exaggerate their symptoms intentionally in order to ensure treatment.

The standard estimate is that between 25-30% of people involved in a Criterion A event can go on to develop full PTSD. Which means that, at the very least, 1,000 people directly affected by the London bombings should by now be displaying full-blown PTSD symptoms. Some of them will doubtless be among the more than 3,000 people who wandered away from the four bombsites without being identified. To date, the NHS trauma response team responsible for diagnosing them has been able to screen only 516 survivors, of whom 178 (a bit above average at 34%) have been offered clinical psychological help. What has happened to the 800 or more other probable PTSD sufferers out there? And what about other survivors who may be labouring through psychological crises without qualifying for a diagnosis? The London Assembly's recent report into the response to the bombings makes it clear that the emergency plan, which focused primarily on the physically injured, left a large group of traumatised victims to fend for themselves, with little information about what they might expect to suffer as a consequence of their experiences.

Almost all who gave evidence to the London Assembly expressed a sense of isolation. Not many were even invited to November's memorial service in St Paul's Cathedral. A gulf seems to have opened up between those who were maimed or bereaved, and the rest - the majority - of the survivors. "Most of those who were severely injured were brought into the system and treated impeccably - physically and psychologically," says Kirsty, a member of King's Cross United, a self-formed group of Piccadilly line survivors. "The big downfall of the whole thing was the people who weren't physically injured but were mentally injured. Obviously, on the day, when there are people with limbs missing, that's the priority, but there should also have been a long-term priority to let people know what can happen to you, and to make an effort to get in touch."

Part of the explanation can be put down to a deliberate strategy. During the King's Cross fire of 1987, groups of counsellors moved in to attend to victims, but this kind of approach has now been discredited. Much of the research into "psychological debriefing" shows that it may not be helpful - indeed, may be actively harmful - to talk individuals through their experiences immediately after a disaster. Blocking, far from a being a bad and "repressive" mechanism, is now commonly seen to be healthy, so long as it doesn't become chronic; and PTSD, it is thought, should be picked up only after symptoms have been shown to persist.

Professor Simon Wessely, a leading PTSD specialist at the Institute of Psychiatry, argues against the tendency to pathologise suffering. "What we've lost in the PTSD discourse is what they knew in the second world war, par excellence," he says, "which is that people's reactions to trauma, adversity, war and terror are determined by the group psychology and not individual psychology. Now we're beginning to remind ourselves that normal people are pretty resilient. They have their own resources; they can maximise their social support. You don't immediately need to involve pointy-headed people like me with our white coats and couches. People are usually the best judges of what they need, and when."

Notoriously, in New York, during the immediate aftermath of 9/11, psychotherapists outnumbered victims, and even began haggling for patients. In Britain, by contrast, trauma culture seems to have moved in the opposite direction, towards a kind of puritanism. There is a powerful scientific and ideological move within the NHS to ensure that trauma treatment should come from a centrally researched and managed programme of CBT and be confined to survivors who meet specific criteria.

Yet a PTSD diagnosis describes particular symptoms only; it cannot quantify a person's suffering and it may not be the best index of how much someone may need help, or what form that help should take. George, a 62-year-old property inspector from north London, echoes Wessely's emphasis on the importance of group psychology. He describes how his journey in the front carriage of the Piccadilly line train out of King's Cross was obscenely cut short in "that bloody tunnel". Afterwards, some survivors - people who would never normally have spoken to, or even noticed, each other on the underground - began to get together in order to complete a different journey. The resulting group, King's Cross United, with its website and pub meetings, acts as an information service for its 100 or so members and a source of emotional support.

George describes King's Cross United as being "as important as a counsellor" for him. He had been just 4-5ft away from Jermaine Lindsay when the suicide bomber blew himself up, killing 26 surrounding passengers. George would later come to realise he had been shielded from the blast by the crush of passengers standing between him and the bomb. He was left, the only person standing in that section of the carriage, dumbfounded by a surreal sense of loneliness in the choking dark. What he found in King's Cross United was release from that isolation.

As he stood in the carriage, amid the screaming and the praying, his first thought had been that the driver must have been killed, which meant that he, too, was about to die. In November, at the St Paul's memorial service, he met the driver - who, on the day, had spent hours underground tending the wounded and dying. By coincidence, they sat next to each other in the cathedral. "I thought you were dead," George told the driver. In turn, the driver told George he was the first person on the train he had spoken to since 7/7. So George invited him to a King's Cross United meeting. "We got the driver there!" George exclaims. "And he's a lovely guy!"

At these informal gatherings, people piece together the details of that broken journey - where they were in the carriage, who was next to them, who they helped, or who helped them. This kind of reconstruction resembles the processes of cognitive therapy. Nevertheless, George found out that the group wasn't, in itself, enough. There were darker and more personal ordeals for which he eventually needed individual treatment.

On the evening of July 7 he had eventually got home, miraculously intact except for cuts in his neck and head, having been told by his wife that his five-year-old grandson Callum knew he had been on the train and was deeply upset. As a result, George described to no one in his family the sounds he had heard in the darkness of his carriage, or the mutilated bodies he saw at Russell Square. "I was very careful about what I said, because I didn't want to upset Callum any more. If I had let it go, I think I'd have let it go completely." So he clammed up, and the following Tuesday, sweating and palpitating, forced himself back down on the tube and in to work.

When George finally broke down in February, the trigger was the sound of Callum crying. It was the crying he had wanted to quieten when he came home on July 7, but it was also the crying he had heard in his carriage that day. He was never screened for PTSD and had found his way to a counsellor by contacting the 7th July Assistance Centre, which provides advice for survivors. What the counselling gave him, in a way that echoed a cognitive approach, was the ability to identify and live with the psychological triggers that had begun to set off his intermittent breakdowns. There were three of these: Callum's crying, the thought of the driver's death (which presaged his own) and, finally, recurrent guilt that, at the age of 62, he should have walked out unscathed while, around him, younger people were maimed and killed.

Guilt is one of the strangest responses to catastrophe survival. It rarely bears much relationship to what actually happened. Survivors can feel guilty if they panicked, or if they remained eerily calm; they can feel guilty if they were too bewildered to help, and even when they performed like heroes. While George was standing bewildered in his devastated carriage, back at King's Cross station, Steve, a 47-year-old inspector with the British Transport Police, was rushing down the escalator, barking orders to a junior colleague and preparing - contrary to protocol - to head alone down the tunnel into the smoke. It is fair to say that Steve was already carrying significant psychological baggage. In his 28 years of service, he has been beaten up and run over, he's confronted a gunman, and been on hand at train crashes and the 1987 King's Cross fire. But July 7 would very nearly break him.

The first down the line to the train, Steve began helping people off at the back, including some whom he would later meet again at the King's Cross United pub sessions. Then he made his way towards the front of the train, and forced open the door into the first carriage, where the bomb had gone off. "I'm holding the seek-and-search lamp and I realise that the people inside can't see what I can see," he says, "and, dear God, I do a quick head count of people who are still alive. There are a number of them I can see who are not going to survive, and I explain that I can't move them for fear of exacerbating their injuries. And I say I'm going to get help. The hardest thing I have to do is to turn around and leave them. There were people in that front carriage who I just could not believe were still alive. And those people so desperately just needed someone to hold their hand while they died, to exit this world with a kind word. Imagine being the only person to go into that. Then imagine the people in there watching as you walk away..."

Having alerted paramedics and other transport police, Steve went outside to deal with his smoke inhalation, and then he went back down into the tunnel again. His actions that day saved lives and earned him an MBE. But he still feels guilt.

"When 7/7 happened, I knew what form my trauma would take," he says. "I reckoned I'd be OK for four to eight weeks, and then I would need to offload it. And at about week six I realised, yes, now I'm going to need counselling. And my counsellor basically saved my sanity."

Steve wanted the same counsellor he had once seen before for a stress-related breakdown, someone he could trust. He was not interested in cognitive therapy, though he recognised that the "reliving" process resembles techniques used by police to help witnesses recall details of a crime scene. "I used to get partial or whole registration numbers from people and they'd go, 'Wow, that's magic.' It's not magic. It's just unlocking a part of the brain. If you do it properly, going backwards and forwards, it's amazing how much detail you can remember."

Steve is still on antidepressants, still has nightmares, but he is ready to part company with his counsellor once next month's anniversary is over. He knows what his triggers are (chiefly a smell of burning), and he knows how to cope.

Kirsty, a 38-year-old designer, was in the last carriage on the Piccadilly line when Steve came through. After the event, like a lot of survivors, she resisted any suggestion that she try to find help. "There is a big thing for a lot of people in my carriage, which is the feeling that 'I'm not worthy to be traumatised, because I wasn't in the first carriage, I didn't hear the screaming, really I should be fine'. For a long time I thought the thing that happened to me wasn't that bad." Until, one night, she cracked. And then she found it extremely hard to find help.

The kind of counselling being offered at the 7th July Assistance Centre, which George had found so useful, didn't tackle Kirsty's specific needs, and she wasn't screened for PTSD until later. Getting back on the tube was for her akin to mini-repeats of the event, but her main trigger was being late, just as she had been on the day. When she eventually lost control last September, it came as a type of dark revelation. "I got this sudden respect for my mind, and a feeling of being kind of separate from it. I've always been an in-control type of person. If you have a bad experience, my response has always been just to get through it. And that's what I tried to do this time, but something came from deep inside that made me wake up screaming one night and wiped me out for two months. And that was way down, totally out of my control, out of my knowledge, like it was another being."

Through a friend, Kirsty eventually found a psychiatrist who specialises in trauma, whom she pays for herself. "The biggest thing he's done is to make me feel normal, and to tell me that what I'm going through is normal," Kirsty says. "He explained how the brain works, and how it gets stuck in a kind of whirring mechanism, like the way the hard drive on your computer can get stuck. I still felt crap, but at least I knew why." Kirsty was looking for other, deeper answers, too, but cognitive therapy isn't equipped to explain things at an existential level, or to tackle the wider human meaning of an event.

For Kirsty, this proved crucial. Like many other survivors, she has become obsessed by a need to understand what happened on July 7 and, most particularly, "What on earth it was that motivated four young men, born and bred in this country, to go and kill themselves with the specific intent of killing innocent people." This conundrum seems to burden all the survivors. Very few appear to feel any anger towards the perpetrators, and few even think of them as murderers; in fact, many survivors will talk about the bombers as if they, too, were victims, brainwashed by non-specific forces. The power of suicide terrorism is, of course, that in the process of killing and maiming, the perpetrator is simultaneously removed from the scene, leaving no target for anger. It is more extraordinary still to think that the suicide bomber may, in this process, even elicit sympathy from some of his victims.

The survivors' animus is almost universally directed at their own figures of authority. "I've found that the majority of survivors are not angry with the bombers, but are very angry with the government, and I definitely feel that myself," Kirsty says. "Maybe it's easier to feel anger against the government. We've voted for them, and they haven't protected us, plus they've lied. But is it actually possible to put yourself in the bombers' shoes, to imagine - which I've tried a million times - what might drive you to do what they did? You just can't, and maybe because you can't, it's too ungraspable to feel angry about."

Nader Mozakka fled Iran as a political activist during the Khomeini regime; he feels fury at the reactionary forces of Islam that inspired the bombers, but even he finds it hard to feel strongly about the individual who killed his wife. Nader, a 50-year-old software manager with two children, is like many of those at the King's Cross United group in this respect - but in no other. Nader is one of the bereaved. He met his wife, Behnaz, while they were at university in Tehran, and together they slipped the net and moved to London, started a new life, raised children and achieved success - she as a charity worker and respected research scientist at the Great Ormond Street Hospital. "Nazy was more than a wife to me," he says, "if you can understand what I mean."

It would be impossible to give a full picture of Nader's grief. Nearly a year on, he remains unable to speak about her without weeping. Where some other survivors have two or three triggers that set them off, Nader has hundreds, from seeing someone who looks like Jermaine Lindsay to tiny domestic details. The only place he can go that doesn't remind him of his wife is an Arsenal game - watching football is the one thing Nader did alone - and even there the crowds make him anxious. Nader was diagnosed as suffering from PTSD not long after 7/7 - although he wasn't anywhere near one of the bombs, he still has flashbacks. Traumatologists call this "imaginal" or "vicarious" exposure to the event. Nader was, he says, "tracing" his wife out of the door that morning, into the underground and on to the first carriage of the Piccadilly line train.

His CBT sessions have gone on much longer than the normal eight to 16 weeks used to treat a trauma and have, at least, helped him reconsider whether his life is worth living. While he was still in Iran, several of his friends were killed by the regime, but that, he says, was to be expected. He doesn't quite buy his therapist's explanation that these deaths may form an underlying pattern on to which has been grafted the great grief of losing his wife, but he does concede they may have made him fearful that others will die and leave him.

But perhaps the clearest example of what can happen to a person as a result of a traumatic event lies in Thomas's experience of therapy. He had not known about PTSD before July 7, and says that during his therapy he discovered a new respect for what his mind could do, even as it was breaking down. Gradually, his emotional responses began to make sense again - with one exception.

During one session in which his therapist tried hypnosis, he realised he was enjoying the sensation of emotion coming back to him, even if that emotion was terrible. And he thought, "How can I enjoy being sad?" And the perversity of this seemed to push him over the edge, into a new, sinister region of consciousness. "In that moment, I had an understanding of how it is possible that you could kill someone and actually enjoy doing that, enjoy seeing someone suffer - and also feel pity at the same time. For me, these were evil thoughts to be having. Before, I used to think I was the good guy over here, and the evil people were over there. But suddenly I realised that we were not that different, because I could understand the same thoughts."

That was enough for Thomas. "I think that thought may be hidden in everyone," he says, "but I don't see the benefit of un-hiding it. If I continued, I would probably uncover more things about myself that I don't want to know."

If you, or someone you know, has been affected emotionally as a result of the London bombings, call NHS Trauma Response, 020-7530 3687 (Mon-Fri); or the 7th July Assistance Centre, 0845 054 7444.