The image is redolent of wars in years gone by. A young, callow soldier, uniform hanging off his adolescent frame, is staring into the camera with a gaze that combines confidence and nervousness at the same time. The photograph, taken in a muddy and remote Afghan compound in 2010, is of John Bryant. At the time he was 18 and Britain's youngest serving frontline serviceman in Afghanistan.

Yet within six months of this photo being taken he had left the Army and was homeless.

His story, say veterans groups, is a particularly stark example of what can happen to servicemen who are not supported when they return to civilian life.

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Before the last election David Cameron promised to enshrine in law a new Armed Forces Covenant that would ensure that "if we are asking our armed forces to do dangerous jobs in places like Afghanistan and Iraq, we are doing all we can for them in return."

But a year into the Covenant, army charities complain that in practice it is little more than political rhetoric.

It is, they say, doing nothing to help soldiers like Mr Bryant – whose problems, although often self-inflicted, are nonetheless acute.

"Once you've done your job in the army – that's you done," says Mr Bryant now. "You're just the riff-raff. It's alright for us to go and put our lives on the line for the country but as soon we need help from the Government and our country – it's like 'nah'."

He is dismissive of the Covenant: "I don't think it makes any sense to be honest because I don't know anyone that it's helped at all. It just seems to be a load of words on paper – just to make a point that it's there."

Mr Bryant, who was brought up in care, had already served more than two years in the Royal Regiment of Scotland when he was sent to Afghanistan a few months after his 18th birthday.

"Afghanistan was crazy," he says. "When I did my first patrol we were briefed to watch where we were walking – we were told there'll be IEDs. Then you look at the maps and they have all these red dots where IEDs have been found – there are thousands of dots – they're everywhere."

Like many soldiers serving on the front line at that time he saw his fair share of pretty awful things. "The first dead soldier I saw was a mine sweeper. We just heard the boom. We didn't really know what had happened and ran up the road. But we couldn't get to him because there were secondary IEDs. By the time we got to him it was too late. He'd only lost one arm but he'd bled out."

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Oddly it wasn't the unpleasantness of the war that made Mr Bryant want to leave the Army; it was the boredom of peace. He knew that after serving in Afghanistan he would be sent back to battalion headquarters with little to do but play computer games and keep up with physical training.

He was young, impetuous, and having travelled abroad wanted to see some more of the world. "I kind of messed up. I wanted to leave because I had a girlfriend. We had plans and stuff. I wanted to go and travel with her before she went to Uni – to do something with my life. I asked if I could take a year out to travel the world because three years of my life had just been spent in the Army.

"I wanted to leave the Army so bad but in a good way."

Despite his age and the fact that he had served time in Afghanistan he still had one year left of his four-year commission – and the Army would not let him break the commitment.

So Mr Bryant claimed to be taking steroids – an offence which would guarantee his immediate discharge. "My platoon sergeant knew it was a lie – because look at the build of me. But I said I need to get out. They discharged me within a week."

He went back to Glasgow but without any family and having split up from his girlfriend he soon began to drift. "I don't like to say how I was surviving when I left the Army but obviously I had to be doing things to get money and that.

"I was handing about with a lot of Albanians at that point. They were pretty nasty people. I knew I had to leave Glasgow. I was going to go to jail or I was going to be dead – one of the two."

With £100 in his pocket he left for London. But he knew no-one and had nowhere to live – ending up homeless.Eventually he was helped by Veterans Aid, a charity which supports, houses, and trains former servicemen regardless of their rank, background or reasons for leaving the army.

The charity has paid for him to train as a scaffolder and have put him up for more than a year at the East London hostel they run. But Mr Bryant is still angry at what he perceives to be the lack of care for soldiers like him who, while often naïve, perhaps deserve more support.

"I joined the army at 16 with no qualifications and left with no qualifications – apart from how to shoot a rifle and a GPMG (general purpose machine gun). I can't put that I know how to fire a gun on my application form for Marks and Spencer.

"There's a really, really big gap from what you do in the Army and what you do next. They're quick enough to get you to sign on the dotted line, give your oath of allegiance to the Queen. But come back from Afghanistan, you want to leave and that's it. There's a medal for you. See you later."

He pauses and adds simply: "We help the Government and the country and the country won't help us back."

Helping heroes: Charitable aims

In recent years the amount of money given by the public to charities for the armed forces and veterans has expanded significantly as a result of the intense focus on the conflicts in Iraq and Afghanistan.

But often the aims of these charities mean that they can only use the money to support a small proportion of ex-servicemen who need help. For example, the military charity Help for Heroes last year raised £46m and its latest accounts show that it has £63m allocated for long-term support.

The charity's stated aims are to support veterans and serving personnel who have been wounded, injured or become sick as a result of serving their country. They also provide support for their families.

The charity says that the total number of servicemen and veterans who have become injured or ill in the two conflicts is more than 5,000. However, this includes all types of illness and injury – where care is often provided by military themselves and where the soldier makes a full recovery.

In the Iraq and Afghanistan conflicts there have been 263 amputees. Ministry of Defence statistics show that 596 soldiers have been classed as "seriously" or "very seriously" injured so far in Afghanistan while a further 226 were injured in Iraq. These figures include personnel who suffered amputations.

Help for Heroes has pointed out that the Confederation of Service Charities works to co-ordinate charitable work across the sector to ensure that aims of different organisations do not overlap. Downing Street has commissioned Lord Ashcroft to report on care for soldiers leaving the Army. He met with service charities this week to investigate gaps in provision.