More than four million men and women from Britain's colonies volunteered for service during the first and second world wars. Thousands died, thousands went missing in action, and many more were wounded or spent years as PoWs. But until now their sacrifice has been largely ignored by the mother country they fought to protect. On the day the Queen opened memorial gates in their honour, Simon Rogers interviewed five such unsung heroes including 105 year old George Blackman who served from 1914-1919 in the 4th Battalion, British West Indies Regiment

George Blackman leaps up, brandishing his walking stick. "Like this," he breathes, imitating the thrust of a bayonet. "Like that," he says, mimicking the butt of the rifle. "I still got the action. I'm old now, but I still got the action."

George is 105. When he was born in Barbados in 1897, Queen Victoria was on the throne and two-thirds of the world was coloured pink.

He points to a scar above his left eyebrow. "That is a bayonet cut on the eye." He touches his hands. "This is from the blow of the rifle butt."

George is almost certainly the last man alive of the force of 15,000 who rushed from the beauty of the Caribbean to the mud and gore of Flanders and the Somme to defend king and country during the first world war. His old comrades are all gone now - the last, Jamaican soldier Eugent Clarke, died earlier this year at 108. When Blackman goes, that will be it.

Sitting in his niece's house in northern Barbados, Blackman is now partially blind and almost deaf. Anita tidies his shirt collar for him as we speak. He is still articulate and energetic, and his fiercest remarks are reserved for England. "I need help but the English government don't help me with nothing," he says. "It's she, she who give me this," he says, gesturing to Anita.

This bitterness has been growing deeper over the years. There was a time when he would have done anything for the mother country. In 1914, in a flush of youth and patriotism, he told the recruiting officer he was 18 - he was actually 17 - and joined the British West Indies Regiment. "Lord Kitchener said with the black race, he could whip the world. We sang songs, 'Run Kaiser William, run for your life, boy'." He closes his eyes as he sings, and then keeps them closed for the rest of our interview.

"We wanted to go. Because the island government told us that the king said all Englishmen must go to join the war. The country called all of us."

Enthusiasm for the battle was widespread across the Caribbean. While some declared it a white man's war, leaders and thinkers such as the Jamaican Marcus Garvey said that young men from the islands should fight with the British in order to prove their loyalty and to be treated as equals. The islands donated £60m in today's money to the war effort - cash they could ill afford.

While Kitchener's private attitude was that black soldiers should never be allowed at the front alongside white soldiers, the enormous losses - and the interference of King George V - made it inevitable. Although Indian soldiers had been briefly in the trenches in 1914 and 1915, Caribbean troops did not arrive until 1915.

The journey to Europe was perilous - hundreds of soldiers from Jamaica succumbed to severe frostbite when their troopship was diverted via Halifax in Canada. Their winter uniforms were left locked up while they froze in thin summer clothes.

When they arrived, they often found that fighting was to be done by white soldiers only - black soldiers were assigned the dirty and dangerous jobs of loading ammunition, laying telephone wires and digging trenches. Conditions were appalling. Blackman rolls up his sleeve to show me his armpit. "It was cold. And everywhere there were white lice. We had to shave the hair there because the lice grow there. All our socks were full of white lice."

A poem written by an anonymous trooper, entitled The Black Soldier's Lament, showed how bitter the disappointment was:

Stripped to the waist and sweated chest
Midday's reprieve brings much-needed rest

From trenches deep toward the sky.
Non-fighting troops and yet we die.

Yet there is evidence that some Caribbean soldiers were involved in actual combat in France. Photographs from the time show black soldiers armed with British Lee Enfield rifles, while there are reports of West Indies Regiment soldiers fighting off counter-attacks - one account tells how a group fought off a German assault armed only with knives they had brought from home. Blackman - who was born to a white mother from London and a Barbadian black father - still remembers trench fights he fought in, alongside white soldiers. "They called us darkies," he says, recalling the casual racism of the time. "But when the battle starts, it didn't make a difference. We were all the same. When you're there, you don't care about anything. Every man there is under the rifle."

He remembers one attack with particular clarity. "The Tommies said, 'Darkie, let them have it.' I made the order: 'Bayonets, fix,' and then 'B company, fire.' You know what it is to go and fight somebody hand to hand? You need plenty nerves. They come at you with the bayonet. He pushes at me, I push at he. You push that bayonet in there and hit with the butt of the gun - if he is dead he is dead, if he live he live."

The West Indies Regiment experienced racism from the Germans as well as the British. "The Tommies, they brought up some German prisoners and these prisoners were spitting on their hands and wiping on their faces, to say we were painted black," says Blackman.

He didn't make friends. "Don't have no friend. A soldier don't got friends. Know why? You believe that you are dead now. Your friend is this: the gun. That is your friend."

At the end of the war, after years of hard fighting, not only against the Germans but also the Turks, men of the West Indies Regiment were transferred to a British army base in Taranto, Italy, where one of the bitterest events of the war would occur - a mutiny. Days were tough there and comprised largely of manual labour such as loading ammunition, or even cleaning clothes and latrines for British soldiers. Blackman, who was not there long, remembers it being hard. "From Marseille, it was seven days to reach Taranto. It is a seaport - all the boats were coming from London with ammunition. We have to unload the boat, the train come and we got to load the train to take the ammunition up the line."

For some of the black troops there, a pay rise for the white soldiers - but not them - was the final indignity. Riots ensued and senior British officers were assaulted. Eventually the mutiny was put down, with one soldier executed and several others given lengthy jail sentences. But the black soldiers were left with a new-found feeling of rebellion.

The immediate result was that the West Indies troops were kept away from the victory parades that marked the end of the war, and hurried home under armed guard. "When the war finish, there was nothing," says Blackman. "I had to come and look for work. The only thing that we had is the clothes and the uniform that we got on. The pants, the jacket and the shirt and the boots. You can't come home naked.

"When we got home, if you got a mother or father you have something, but if you're alone, you got to look for work. When I come I had nobody. I had to look for work. I had to eat and buy clothes. Who going to give me clothes? I didn't have a father or nobody. Now I said, 'The English are no good.' I went to Jamaica and I meet up some soldiers and I asked them, 'Here boy, what the government give you?' They said, 'The government give us nothing.' I said, 'We just the same.'"

And that's when Blackman disappeared off the veterans' radar. Travelling around South America, he worked as a mechanic in Colombia, before retiring to Venezuela to live with his daughter until the Barbados government helped to bring him home earlier this year.

As a Barbadian living in Venezuela for decades, he was not entitled to a pension there. The Barbados government (in the form of one dedicated civil servant) is still processing his application for one in his home country. And from the British? Nothing.

The empire changed when Blackman and his comrades returned from France. The soldiers who emerged were so politicised that island governments encouraged them to emigrate to Cuba, Colombia and Venezuela. Those who returned to their countries altered everything. Gunner Norman Manley, who had seen his brother blown apart in front of him during the war, eventually took Jamaica to independence, becoming its first prime minister in 1962.

A secret colonial memo from 1919, uncovered by researchers for a Channel 4 programme on the Taranto mutiny, showed that the British government realised that everything had changed, too: "Nothing we can do will alter the fact that the black man has begun to think and feel himself as good as the white." In a sense, history was rewritten. That meant no celebrations, no official acknowledgment.

For George Blackman, the situation has become even more simple. "England don't have anything to do with me now. England turned me over." He opens his eyes - they are almost blue. "Barbadians rule Barbados now."