It's a fascinating legend, which makes Kent the birthplace of England and the English language...

The tale of how two Germanic warrior brothers sailed to Thanet after the collapse of the Roman Empire and fought as mercenaries, before turning on the British and defeating them at Blue Bell Hill, thereby becoming the first Kings of Kent.

From there the Saxons became the dominant culture, driving out the British tribes, and their language became the foundation of modern English.

There's only one problem with the first part of the story, it's a lie – and if Hengist ever existed at all, he was more likely to be a pig farmer who came over in a rowing boat.

Rainham author John Lambshead's new book, The Fall of Roman Britain, And Why We Speak English, looks at the rise of Saxon culture after the departure of the Romans in 410AD, and the historian provides a fascinating insight into the catastrophic story of post-Roman Kent.

It might come as a disappointment for fans of blood-thirsty axe-wielding Saxons, but it's good news for fans of dystopian disaster movies. Or to misquote the great Bill Shankly, the situation wasn't life and death on the battlefield, it was much more serious than that.

"The evidence is overwhelming in southern England that it was a disaster zone," explains John, a retired research scientist who worked at the Natural History Museum. "Cities were abandoned, we lost the art of the potter's wheel. The descendants of Romans at Cadbury Hill in Somerset were raiding graves to get urns to use as pots."

"In Kent it starts with Constantine III (the Roman general who declared himself Western Roman Emperor in Britannia in 407). He stripped London and the surrounding regions of their gold and silver. Constantine III was trying to strip the South East of troops and taking them to the continent to fight other Romans.

"The minute he went there was nothing left in Kent. There was no money, no authority, no civil servants – it's the same as no one putting any food into the supermarkets. I think it was Lenin who said every society is only three meals away from chaos."

With the removal of the 'elite', all those who made money from the elite disappeared from the scene, and Kent – which had been one of the most civilised and prosperous areas of Roman Britain – was hit hardest of all.

"Kent's always been important because London has always been important – and Kent stands between London and the continent," adds John. "If I look out of my window and I can see housing estates on what used to be the rural village of Rainham, for the same reason. Kent would have been one of the richest places in Roman England but it meant when the catastrophe came it was hardest hit.

"All the money stopped coming in from Europe. It just stopped like a hammer coming down. "There was no one in the area even making iron nails, and even writing disappeared."

"We've lived through very little comparative catastrophe. Even World War Two wasn't really a catastrophe, because at the end of it the population was not dramatically affected. We're not talking about bombs dropping on London, we're talking about grass growing in the streets. We're not talking about looting – there was no-one to do the looting. It's more like the Day of the Triffids."

And so into a power vacuum and cultural wasteland, rode – or more precisely, rowed – the early Saxons from Northern Europe, wielding hoes and other farming tools with serious intent.

The revised view of British history is that the Saxons, without invading militarily, soon grew to become the dominant force, subjugating the existing British culture in the process.

But John suggests a further refined view is that the process happened far more slowly – over centuries – and part of the evidence for a slower rise to power comes from fascinating DNA evidence.

In his former role as a research scientist, John specialised in marine biology and evolution, and he was able to use studies carried out with advanced 'high-throughput' or mass-screening techniques, to bolster his argument.

"It's a massively powerful tool," he added. "When we looked at German migrants coming into Britain, we found it didn't match the history.

"We knew German migrants had been coming into the country from around 425AD. We know the Germans, or the English, started arriving but we're not talking about an invasion. We're talking about families arriving in rowing boats.

"When they did the big DNA study, the conclusion was that actually mixing between British and Germans didn't start until 800AD. The other thing we've found is there's not too much DNA – it's 10 to 40 percentage and the rest is British."

After centuries of a slow rise, and population expansion, John suggests that the real take-over of Saxon culture happened between 800 and 900AD, in a phase known as the Saxon Shift – when successful Saxon communities begin to take over larger areas of land, kingdoms begin to form, and the big medieval battles begin.

He believes until that point there had been limited interaction between Saxon and British communities, but once the Saxons established themselves as the rising power, the British began to be subsumed into their culture – and the Saxon, Old English, language became the dominant tongue.

That's when names such as Gillingham, begin to appear - 'ham' being Old English for homestead, which in this instance belonged to the people of 'Gylla', an Old English personal name.

"You get this shift into land ownership," adds John. "All the names we think of come from this Saxon Shift, and this explains why we speak English. The same thing happened in Iceland. Iceland was colonised by Vikings but they took Gaelic slaves. Modern Icelandic language is completely Norse and has no Gaelic.

"The same thing happened to the British – they became a kind of lower-class, or unfree people. "In that situation, if you're a Gaelic mother and your child was fathered by a Norse father, you want your child to grow up Norse. In the same way, a British mother would want her children to grow up as Saxons."

The reality of that class difference is hammered home by the Saxon word for the British – the 'Welsh', a word which meant both 'slave' and 'foreigner', two concepts that the Saxons didn't see as any different.

In short, it paid to become Saxon.

"Once you get that set-up there didn't have to have many Saxons initially to take over," says John. "They were recruiting the whole time."

"It was a gradual process. They weren't a race or nationality; those are modern words. What they were was a culture. "

"They didn't need big armies. They arrived and encountered what I'd call the fag-end of a culture, with little villages with subsistence farmers.

"The Saxon migrants were a primitive society but they were a functioning society and they could get anything they needed from the continent. Word might get out that for example that there was a good living to made in Kent as a smithy. He might be Frankish or anything but once he arrived he was Saxon."

Of course, as a national story, the narrative is slightly lacking in epic battles and dashing heroes. It's only after the Saxons have been around for a while, in the 8th 9th and 10th centuries that tales begin to be told of a more heroic past.

Texts appear such as Beowulf, which tells the story of an ancestral hero from Scandinavia, while the Anglo Saxon Chronicles of the 9th century mention the figures of Hengist and Horsa – who reportedly battled the British at Blue Bell Hill in 455.

While Horsa dies, the chronicle says Henigst took control of the kingdom, before fighting the British again in 457 at Crayford, "and there slew 4,000 men". In the year 473, the final entry in the Chronicle mentioning Hengist, he and his son Esc are recorded as having taken "immense booty" and the Britons "fled from the English like fire".

But John – along with most modern historians – says Hengist and Horsa were probably as real as the marsh-lurking troll Grendel from Beowulf.

"It's a complete myth," he says. "You can't believe anything written by Saxons about the time before 800AD, because that's when they started writing.

"They were inventing a new history. Everyone did it – the Welsh chieftans claimed they were descended from the Magnus Maximums – a Roman general who revolted, and the Frankish Kings claimed to be descended from Roman emperors.

"Think about the poor Saxons who started these kingdoms. What are you going to say? 'My great great grandfather was a pig farmer from the northern Rhine who came over in a rowing boat'? The Saxon kings claimed descendants from the gods like Thor and Odin – it's inventing the past."

And John – a dad of two, with two grandchildren – believes there was no battle at Blue Bell Hill or Aylesford between the Saxons and British.

"We're not talking about an invasion. We're talking about families arriving in rowing boats..."

"I just think it's not true. I would say I would challenge anyone to provide any evidence that it happened. The authors of the Anglo Saxon Chronicles were writing around 800-900AD; how would they know what was going on at Blue Bell Hill in 400AD? We can't trust that early history.

"They were trying to give them a mythologised past, where they did great things. It gives you no social standing to say my grandfather was a pig farmer. It's better to say my grandfather was a warrior who defeated kings."

In other words, in an effort to hog this joint for themselves, those sons of pig farmers were telling porkies – but they were telling them in a language that would change the world.