JOHN MATTHEWS, historical advisor to this summer's epic film, KING ARTHUR, reveals why everything you thought you knew about the legend will be turned on its head

The legend of King Arthur is one of the most enduring and romantic chapters of our island story and this summer, it's going to be one of the most talked about, too. A huge new multimillion-pound production about the King is one of this season's blockbusters and takings will, if all goes to plan, match the movie's colossal scale. Less certain, though, is what the film's global audiences will make of its surprising take on this old story.

To most of us, Arthur is either misty folklore or a rollicking good yarn written in the Middle Ages. Some will have heard how the young Arthur drew a magical sword from a stone to prove himself the rightful king. Others will know of Merlin, the great magician, who constructed a Round Table at which a band of heroes would gather. Think of the gentle animations of Disney's The Sword In The Stone, or Richard Harris singing about Camelot, where even the weather was perfect. ExcaIibur, John Boorman's 1981 movie, was a lot more faithful to the story, but still had all the romantic trappings. And who can forget 1975's Monty Python And The Holy Grail?

The point is, all these versions focused on the medieval legends of Arthur and his knights. The new movie, written by David Franzoni, who also wrote Gladiator, gives us an Arthur very much for our own time. Darker, more barbaric, full of action and humour, it paints a savage and realistic picture. Here, there is no many-towered Camelot or knights rescuing damsels in distress. Instead, Arthur is just a man. He has been a professional soldier for most of his life, in an era when life often depended on strength and cunning rather than chivalry or magic. But most intriguing of all is that Arthur is a Roman - and it's this fact that brings the movie nearer to the historical truth than any film before.

Without giving too much away, the story runs like this: Arthur Castus (Clive Owen) is a Roman officer with a British mother. He leads a group of heroes, the Knights of the Round Table, including Lancelot (loan Gruffudd) and Bors (Ray Winstone), who come from a far-off land called Sarmatia - roughly between the Black Sea and the Caspian Sea. Shipped off to Britain to fight in the Roman legions, we see them fighting against the Woads, cunning and ferocious adversaries from north of Hadrian's Wall, led by Merlin (Stephen Dillane), who is not a twee magician but a potently mysterious shaman.

Arthur and his men are awaiting their discharge papers, but a messenger turns up with 'one last job' for Arthur and the knights. Their task is to rescue a Roman family from the wild lands north of the Wall. Among these fugitives is a boy, Alecto, who is a favourite of the Pope and, we are led to believe, very possibly a future pontiff himself.

Along the way they rescue Guinevere (Keira Knightley), who is a Woad. She falls in love with Arthur and attempts to bring her own people over to his side to fight a common enemy - the Saxons. These cruel and savage warriors, led by the fearsome Cedric (Stellan Skarsgard), intend to capture the Roman family and hold them to ransom, while burning and pillaging much of Britain along the way.

Not that I knew very much about the storyline when I first got the call, early last year, asking if I would act as historical consultant. Having studied the Arthurian legends for over 30 years, in all that time I had never seen a Hollywood movie that came anywhere near to the historical truth. I set off for Hadrian's Wall, where I was to meet producer Jerry Bruckheimer (Top Gun and Pirates Of The Caribbean), director Antoine Fuqua and David Franzoni. I waited in the snow and watched the road, but they arrived, in true Hollywood style, by helicopter. At one point, as I marched in front of the group, I looked back to see them strung out along the Wall, every one of them talking into mobile phones.

Some months later, I went on location, a few miles outside Dublin. 'Dead' bodies lay in piles. The crew walked around with buckets full of spare hands, fingers, arms and feet. But there, beyond the tents containing the film’s huge wardrobe, stretched two and a half miles of recreated Hadrian’s Wall, complete with Roman fort and Dark Age village. I touched the Wall’s great gates, so heavy they had to be pulled open by horses. And I climbed to the top of the rampart and looked out across the fields to the hill that would become the site of Arthur’s battle against the Saxons. It was like stepping back through the centuries; I felt what it must have been like to stand inside the Arthurian legends, to be alive at that time. And, to top it all, with a script that was realistic.

The script’s central premise is not new – the idea that Arthur was Roman has been around for decades – but what gives it an additional spin is the inclusion of the group of Sarmatian warriors. These nomadic people had a run-in with the might of the Roman army under leadership of the Emperor Marcus Aurelius in the 2nd century AD. Having defeated them, but admiring their courage and fighting skill, the Emperor drafted the survivors into the legions and posted them as far away as possible.

The furthest northern border of the Empire was Britain, and here the Sarrnatians were sent Their headquarters were in a fort at Ribchester, in Lancashire. Archaeological evidence from this site shows that they formed a close-knit community, retaining many of their social and religious rites. From the fort, they were posted to locations around the country, including Hadrian's Wall. Their leader at this time was Lucius Artorius Castus - Arthur. So we have the germ of a story - a leader named Arthur defending the Wall against the northern tribesmen and probably encountering a new enemy - the Saxons.

These people, from Fresia, Germany and Jutland, were invited into the country as merce¬naries by Celtic warlords. Once here, the Saxons saw Britain as easy pickings. Without Rome's armies, and with many of the Celtic tribes fighting each other, boatloads of warriors began raiding the country. It is at this point that the story of Arthur really begins.

What little evidence we possess mentions a charismatic leader of part Roman, part British blood, who welded together the feuding Celtic tribes into a force strong enough to drive the Saxons back to the coast. We cannot say for sure that this leader was called Arthur; what we can be certain of is that such a person existed.

But what of the Sarmatians? We know that they venerated a sword stuck point down¬wards in the earth; and that they fought under a windsock-style banner with the head of a dragon emblazoned on it. The wind entering through the mouth of the beast made a roaring sound as they rode into battle, terrifying their enemies. In later medieval stories, Arthur is often depicted as carrying such a banner.

This is not the only way the Sarmatians may have influenced the leg¬ends of Arthur. They had stories that are very familiar - of a Lady of the Lake; a shamanic leader not unlike Merlin, and a leader wounded in battle who commanded one of his followers to throw his sword into a lake. The same story is told of Arthur, who, having been wounded in battle, commanded Sir Bedivere to throw his sword Excalibur into the lake from which it came. Even the name Excalibur may have originated from Sarmatia. A local tribe famed for their skills in smithcraft, were known as the Kalybes. The oldest name for Excalibur is Caliburn, a word that originates from chalybus (steel) and ebumus (white).

It seems more than likely that stories of the Sarmatian warriors still endured long after they were gone. And the wandering bards and storytellers of the Celtic peoples would have been familiar with these tales. If this evidence were not enough, some of the place names around Hadrian’s Wall have a familiar ring. One of the the major forts is called Camboglanna, a name similar to Arthur's famous capital at Camelot. Legend also tells that the wounded king was carried to a place named Avalon. Less than 20 miles from Camboglanna was the fort of Avalanna.

In recent months, a new piece of evidence has come to light. My wife and I discovered a poem dating to within 200 years of Arthur's time. This poem speaks of a leader born of two nations, of Rome and Britain, who fought against a terrible enemy, on the Wall. The name of that leader is Arthur. I still remember the excitement I felt as I read it. Needless to say, I showed it to Antoine Fuqua and Clive Owen, who were both delighted This was proof that the Arthur we were working to bring to life could well be the most accurate anyone had yet come up with.

It was my job to answer hundreds of historical enquiries and to keep a wary eye out for conflicts between the real Dark Ages and what the imagination of a Hollywood designer might come up with. We did not always agree - but, then, this was a big-budget Hollywood movie, not a historical documentary. Having long since decided that there would be a Round Table, we were delighted to find that the Sarmatians liked to eat at circular tables. The great love story of Lancelot, Arthur and Guinevere is also present, though it is much more low-key.

Such attention to detail could hardly fail to rub off on the cast. Keira Knightley - full-blown tattooed warrior woman - relished the part and trained fiercely every day. Clive Owen admitted that he had fallen off his horse during his training for the part. Actor loan Gruffudd remembered hearing the stories of Arthur as a child growing up in Wales, and was fascinated to learn about the Sarmatian theory.

Taken together, it is the realities, dark and brutal, that make this new movie so powerful. I remain convinced that Arthur and his knights were real people, and that something not unlike the struggle portrayed in the film really took place. Working on it, seeing it brought to fruition, means that every word I write about the Arthurian story from now on will be touched by memories of its newest incarnation.

King Arthur opens on July 30. John Matthews' King Arthur, Carlton, £14.99, and Merlin, Mitchell Beazley, £16.99, are published this month.