Tom Hurndall, 21, was a young, compassionate man when he went to Gaza in 2003. Months later, while he was rescuing Palestinian children from gunfire, he was shot by an Israeli army sniper. On the eve of a Channel 4 film, his parents tell of their anger, loss, intense grief and political awakening as they sought to bring his killer to justice

This story begins with an ending. On 11 April 2003, Thomas Hurndall, a 21-year-old photojournalist, was shot in the head in Gaza by a sniper from the Israeli army.

Tom was a brilliant, intrepid young man, driven by an energetic morality, a wish to make a difference in the world. The shooting left him with unsurvivable brain damage, but he clung to life - against the odds - in a coma, for nine months.

While he lay dying in Tel Aviv and later in the Royal Free Hospital in Hampstead, north London, his parents, Anthony and Jocelyn Hurndall, took on a heroic struggle against the Israeli army. They were determined to seek truth and accountability at all costs. They had no idea how hard this was going to be.

The Israeli army appeared to view Tom's death with indifference; there were no plans to investigate the shooting, interview witnesses or go to Gaza. Nor, at first, were they willing to meet the Hurndalls. Their claim was that their soldier had fired at an armed terrorist. Tom, dressed in an orange jacket (a known sign for peace workers), was unarmed. What's more he was shot while rescuing Palestinian children.

Faced with lies and silence, Anthony, a commercial lawyer, did the only thing he could: he took the case on himself. It was his meticulous investigation that led to the prosecution of Bedouin sniper Sergeant Wahid Taysir, who got eight years (the longest sentence ever received by an Israeli Defence Forces soldier for shooting an unarmed civilian in the occupied territories). Vengeance was not Anthony's motivation. He wanted only to find out what had happened. But the verdict was, in the bleakest way imaginable, a personal victory.

Jocelyn has told her story in a beautifully written, uncompromising book, My Son Tom (Bloomsbury). More riskily, Channel 4 has turned the story into a drama to be broadcast tomorrow: The Shooting of Thomas Hurndall, written by Simon Block and directed by Rowan Joffe. Stephen Dillane plays Anthony and Kerry Fox plays Jocelyn. The film manages to be neither sensational nor sentimental - quite a feat.

It is its understatement that overwhelms. Anthony and Jocelyn divorced several years before Tom's death and the film conveys the strained decency of their relationship and their admirable transcending of differences.

The three surviving children - Sophie, Bill and Fred - are carefully characterised, and what the film spells out is that the shooting of Tom was a shattering of the entire family. Each of them took the impact of the sniper's fire. But I wondered if the film might have taken some emotional liberties too: Anthony is seen to be all head, Jocelyn all heart. And they are presented in rivalrous stances, as if we are being asked to choose between them. Was it really like that?

I met the Hurndalls separately: Anthony first, on a cold autumn day, in his offices near the Bank of England. He is tall, with a shambling gait, and is absolutely direct. You don't feel any slipperiness, any side. I liked him at once and admired his ability to talk with clarity about complicated issues. At the same time, he often left sentences unfinished as if he lacked the will to complete them. I noticed how often he leant back and shut his eyes when speaking, as if closing his eyes would have the opposite effect - make it possible to see.

For Anthony, the film is a 'further memorial' to Tom. But what matters most to him is that audiences should understand that what happened was 'not a freak accident, but a product of a policy that the Israeli armed forces were adopting in Gaza'. There is barely suppressed outrage in his voice as he remembers the British government's failure to protest when Tom was shot: 'The government viewed Israel as a close ally who they did not want to put out in any way.'

It was only when a Tel Aviv bar was bombed by two British Muslims three weeks after the shooting of Tom in 2003 that Anthony became aware of how skewed the British government's attitude could be. 'Jack Straw expressed deep sympathy to the Israelis and promised to put all the resources of the British government at their disposal. This was our government taking responsibility for two people who were not employees of the British government, merely two citizens of Britain who happened to be in Israel.'

But when their own British citizens (Tom, along with Iain Hook, a UN worker shot by an Israeli sniper in November 2002, and James Miller, a documentary-maker shot by an IDF patrol in May 2003) were attacked by Israeli soldiers, there was no outcry (no ministerial interest at all, beyond a standard request, from a junior level, for a proper inquiry). 'They were shot not by people for whom the Israeli government had no responsibility but by their own soldiers. That, for me, was outrageous.'

Anthony approves of the film and thinks Dillane got a lot of things right. But he resists the idea of himself as the 'dry lawyer'. He feels a 'little put out' at a scene in which he is in a hurry to get back to the office while Jocelyn waits devotedly by Tom's bedside: 'It was not like that.' He finds Dillane's Anthony 'more confident than I am. I am much more uncertain about things.' He admits he can switch gear into 'detached' mode and adds: 'I do live in my head.' But he wants it on record that 'there is a lot of emotion there'.

I don't doubt it. The way he uses Tom's name as emotional shorthand reveals the strength of his feelings: 'When I heard that Tom had been shot, I felt angry and worried, but already I knew that if this had happened, this was Tom doing something which was Tom. When we heard he had been going to rescue children - that was absolutely Tom. He had this strong, protective urge - a tremendous feeling of caring about putting things right.'

It was Tom's need to put things right that became, for Anthony, an unspoken direction, an imperative. He remembers it like this: 'Tom, you are lying there. This is what you want. This is what we are going to do for you, Tom - with you.' That 'with' is affecting because the more accurate word would have been 'without'.

Anthony describes Tom as a little boy who was 'troubled', although 'outgoing', and 'determined to do his own thing'. He 'hated arrogance', intellectual or otherwise. He was much loved because he was 'always there for people. He would talk to anybody, on any level, about anything.'

Tom was 15 or 16 when his parents split up. 'Being the angry teenager he was, we didn't talk for a while,' Anthony recalls. But football (Arsenal) and photography were shared passions. And by the time Tom left England in 2003, 'we had re-established quite close bonds. We were still a little aloof but much closer. We discussed his preparations for the trip. I remember talking about what he was going to do to keep safe.'

Tom also discussed the trip with his 'very closest friends, who happened to be Jewish'. But his attitude would have been: 'I want to find out for myself.'

It is painful listening to Anthony piecing together the past, trying to reconfigure his relationship with Tom before he died. I suggest that it was remarkable that he and Jocelyn managed to work together with such dignity. 'There have been enormous stresses,' he admits. 'Don't be under any illusions about that, but I think we are now closer in many ways.'

Before I leave, I hear myself asking - I hadn't meant to put it so baldly - how much Tom's death has changed his life? 'The whole experience has changed my view of life and of what is important,' he answers. He describes being 'in the City, in a bar, seeing people have a good time - it seems completely unreal'. He admits he has lost the taste for 'life's pleasures'. 'Work is what matters.'

Anthony's work now is to create a forum for people to solve their legal difficulties outside the court. He finds, in this good-hearted project, an af finity with Tom. But he remains hard on himself. He believes that because of Tom's death: 'I am a lesser parent. I don't take the children on holidays.' He bought some golf clubs: 'They haven't been out of the cupboard.'

He feels that Tom's death has made him more spiritual: 'I used to read to Tom in intensive care. I spent hours with him. I read the Koran for the first time and Taoism. It was a time for contemplation, working out what was important.'

Gaza, he explains, was 'so dramatic and powerful. The best journalist cannot portray the reality of what is happening. What is an Israeli-only road? What is a checkpoint?' He believes we have 'so little idea of what is happening'.

Grief, he recognises, can make you see more keenly. It has also made the political personal: 'I remember saying to someone [about Gaza and the Middle Eastern conflict] the Israelis can't say the problem is only theirs. This is now our issue.'

Jocelyn Hurndall lives in Tufnell Park, north London. She has a radiance that is immediately attractive. But faces carry their recent emotional history and her grief is as indelible as a watermark. She wonders whether we might sit in her garden. It is a is lovely, peaceful, idiosyncratic place - a shrine of sorts. I exclaim at the tiny, weeping silver birch at its centre: 'It was planted for Tom,' she says. 'It's going to make me cry.' 'Don't,' I say. 'It's doing your weeping for you.' I feel an immediate rapport with this woman whose emotions, five years on, are still so close to the surface, so available to her.

In her book, she wrote: 'People know about death in the Middle East... it is one of the things that gives life there its vibrancy. In the West we are not forced to look death in the face in the same way - we barely acknowledge its existence. We take our lives very much for granted.' Tom's death has changed that permanently: 'I never take my life for granted.'

She tells me: 'Tom valued the moment, the meaning of things. I respect that. It is what I have learnt from him.' And after his death, she learnt that being able to 'meet the moment' was an essential part of grieving too - not running away even from the most agonising details: the scent of Tom's clothes, the look of his handwriting.

We talk about gardening, how it insists you live in the present (but with an optimistic eye on the future). Her hands, she says, have been roughened by it, but it has helped her heart. Being able to garden is emotional progress for her. 'For a long while I could not look at anything beautiful - lichen, for example, that has taken a lifetime to grow on a stone. I couldn't look at the beauty of this world without thinking about what Tom had lost.'

The film conveys the profound alienation after Tom's death - the shock of being pitched from London into a Middle East war zone. Isn't grief itself like this; doesn't it bring with it a feeling of being in a foreign country, an attendant unreality? A sense of not being at home in your own skin? She agrees. 'Back then, even being with a group of friends felt foreign. You weren't able to give anything back - and that was shocking.'

One insight in her book is that a family is seldom united by grief. Mourning is an individual thing, a lonely road. I can see (as Jocelyn herself recognises) that her emotional articulacy might not always have been easy for her family. And it must have been hard for her too. Looking back, she finds Anthony's grieving 'completely different' from hers because 'it was so far under the surface'. She says: 'My brain was shot. He seemed to be able to use his.' Her son Billy was different again. He coped by completely rebuilding his little brother Fred's room after Tom's death, an awesome job for a boy with no formal carpenter's training. Jocelyn is still so proud of this that she takes me to inspect his handiwork.

I wonder how Jocelyn feels now about allowing a child, albeit a grown-up child, to go to a war zone. 'When Tom first said he was going away [Iraq was his initial destination], I froze. I could not speak when I found out my challenging, extremely bright son was planning to do this. I felt: how could he put us through this?'

It was a long time before she was able to respect his decision. Now she believes 'we learn so much from risk takers', from people not content to 'sit on their comfortable sofas'. She remembers Tom asking her 'why a 21-year-old on the other side of the world should live a much more difficult life than I do in London'. She salutes his thinking: 'He saw acceptance of difference as one of the answers to the universe,' and she agrees with him: 'Inclusion is essential - on climate change, on banking crises. We need to think in a more global way.'

Since Tom's death, Jocelyn no longer works as a head of learning support at a state school. She is, instead, the director of the Middle East charity Friends of Birzeit University, which raises funds for academic scholarships for Palestinian students. 'It is to do with education for an oppressed group who experience barriers to learning,' she says. Her daughter, Sophie, has also gravitated towards charity work; in her case, Map (Medical Aid for Palestinians). She has become an exceptionally effective campaigner (the importance of her role in the family has been written out of the film).

Jocelyn hopes the film will make audiences 'abandon an emotional response to the conflict and make an objective attempt to open their minds to what this is really about. Highly intelligent people often lose all their logic about this conflict.' It is interesting that just as Anthony wants me to know that he is emotional, she wants me to know she can be logical. But both Hurndalls want people to know what is happening in Gaza. Her eyes blaze as she describes the 'wanton destruction of people's homes'. She wishes she could bring politicians in 'by the busload' to see what she has seen.

And yet it is clear that Jocelyn will never find it possible to abandon the emotional response to the Middle East conflict. 'The life-changing moment for me was when Fred, who was only 12, was interviewed by the BBC. "How do you feel?" they asked him. He was standing next to Tom's blood on the ground. He was like a ghost. He said, "They shot my brother, he was doing something good, he was rescuing children."'

She tells me that part of Tom's legacy is the realisation that how a story is told is crucial - words matter to her, as they did to him, tremendously. It is this that suddenly reminds me to ask her about the Israeli judge who passed sentence in words so unjudiciously lyrical they take the breath away. She knows 'nothing about him', she says, but continues to cherish the words he chose. This was his summing up: 'Sergeant Wahid Taysir caused a soul to leave this world. He spilt the blood of a young man in the bloom of youth, causing the loss of an entire world. When that young man was alive, there was no one else like him, and there will never be anyone like him again.'

• The Shooting of Thomas Hurndall will be broadcast on C4 Monday October 13 at 9pm

• Friends of Birzeit University campaigns for the educational rights of Palestinian students. For more information, visit fobzu.org or to make a donation, call 020 7832 1340