IF YOU say Henry VIII to most people, the iconic image that is likely to spring to mind, will be the magnificent full-length portrait, painted by his contemporary Hans Holbein. In it, Henry looks impressive and imposing: jewels on his chest, codpiece thrusting forward, arms akimbo and his eyes glaring out of the canvas.

However, according to one leading Tudor historian, like many people across the ages, we have been bamboozled by this stunning image. Indeed, Derek Wilson from West Buckland goes so far as to say the portrait is probably the most effective piece of propaganda in the whole of English history.

"This is Henry as he wanted to be seen," says the author.

"The reality was rather different. When that portrait was painted that apparently magnificent man was fat, balding and a semi invalid. He had just fairly narrowly survived a major rebellion in the country. He had been 28 years on the throne and he had no male heir. His own illegitimate son, his fall back heir, had just died. He was on his third wife and he was actually staring failure in the face."

Henry, he contends, strikes that famous domineering pose to suggest he was a splendid and powerful king.

"This is not the case," says Derek. "We have been deceived."

In latest book on the Tudors, A Brief History of Henry VIII, Derek, puts forward the view that Henry VIII was a man who lived in the shadow of his own father. He was haunted by the achievements of Henry VII, a fine king who had established peace in the country after the Wars of the Roses. Henry VII won his crown in battle, saw off rebellions and was a man of considerable stature.

"Now Henry VIII simply doesn't compare," says Derek. "He never led an army in battle. He was present at the odd siege but that doesn't really count."

The stunning piece of art by Holbein has its origins in the Whitehall Cartoon, a large fresco created for the walls of the Privy Chamber in Whitehall Palace. It shows Henry and his then wife, Jane Seymour, with father Henry VII and mother Elizabeth of York. They are grouped around a stone plinth with a Latin inscription.

"You might, I suppose, have expected that inscription to say how marvellous the Tudor dynasty was," says Derek. "What it actually says is: 'Here you have two kings. Which is the better one? Well it's me. I'm better than my father'."

This, feels Derek, highlights Henry's great insecurities.

"Here's a man in his 40s still feeling the need to show he is escaping from his father's shadow. He obviously isn't. He's saying: 'Well yes my dad may have given peace to the country but I've purified the church'. It was the only thing he could think of that he had done that his father hadn't done."

Derek believes there is a lot that needs to be unpacked about the famous king in this, the 500th year since his accession.

If you saw the recent BBC romp, The Tudors, you would be forgiven for thinking that Henry was something of a stallion between the sheets.

"He was no great stud apparently," contends Derek. "In fact he's almost certainly, by the time this portrait was painted, suffering from impotence. This great stud – how many children did he have? Legitimate and illegitimate. Not many. Now you've got Francis I who could have populated a small village with his offspring. The same is true of Charles V, his contemporary in Spain."

By the standards of the day then not a sexual athlete, we might conclude.

"I think there were possibly two reasons for that," adds Derek. "There might have been some physical disability but also mounting psychological anxiety. Of course his weight, that didn't help. I mean as a woman, can you imagine it? Would you go to bed with this man?".

Of the glamorous BBC series, which was neither purely fictional or a historically accurate account, Derek says: "I find myself bewildered by that particular series. Some of the facts they got wrong and must have deliberately got wrong. I simply can't understand why."

Isn't though Derek being a little harsh on Henry VIII? After all he was instrumental in bringing about The Reformation in England. The country he inherited in 1509 was not the country he left in 1547. England was emerging by Elizabeth's reign into an important maritime power and a leading protestant nation of Europe.

"The tendency is to think therefore: 'Oh well, he must have been a really great king to have achieved all that'. But a lot of it was not his doing at all. Why did we become a protestant nation? Well because he wanted to get rid of his queen and this led to a controversy with the pope that escalated and escalated until Henry decided: 'Okay well I am now the head of the church'."

Similarly, contends Derek, although the Royal Navy was founded in Henry's reign this was not due to the king's vision to make England into a maritime power but more to do with his dicey relationships with foreign powers.

"He was dead scared of invasion. He develops the navy and studs the coast with fortresses because the pigeons could be coming home to roost."

Derek has another book about to hit the shelves. This one about another famous tyrant of history, Peter The Great of Russia. It's interesting to make comparisons between the two leaders.

"Henry didn't give a fig about the greatness of England while Peter was desperately concerned to build a powerful Russia. He had a belief for his country. Henry wasn't a man of vision at all."

Of the two tyrants, which one would he rather be friends with?

"I think Peter was undoubtedly the greater man. If you wanted to be in his close entourage you had to be a remarkably fine drinker apart from anything else. He could drink everybody under the table. He was a man of huge appetites, again similar to Henry VIII. I wouldn't have matched Henry, though, in a drinking contest against Peter."

How these two rulers dealt with their enemies is quite revealing, contends Derek.

"When Peter was faced with rebellions he put them down ruthlessly and was quite prepared to go into the marketplace with a sword or axe and carry out executions himself. Henry, I don't think ever stared death in the face at all. What he did when somebody had fallen from his favour was he simply withdrew from them."

The most famous example, from the king who was married six times, was when he abruptly withdrew his presence from his second wife, Anne Boleyn.

"That I think is the biggest single blot on Henry's reign: the way he treated Anne Boleyn. An appalling story. I've never really got to the bottom of it. I don't think that anyone has got to the bottom of it. Exactly why. It wasn't just that he wanted to get rid of his wife but to actually have her killed. Not only her but several men of his own court and even his close friend, Henry Norris. What's behind that? I think he was losing it by then. He was becoming paranoid."

Having unveiled Henry's insecurities and the motivations in the book, does Derek feel we learn anything from his reign?

"It teaches us that we must always be careful of tyranny. Even in a democratic system of government you can have tyrants. Let us always be aware of those in whom we invest power, that they are people who have some vision for our well being, not just there for their own ends. Every time we get an indication of sleaze in high places, we ought to think: 'Those who have our destiny in their hands, have they really got our wellbeing in the forefront of their minds? Are they in it for themselves, for their own power, their own gratification?'."

● A Brief History of Henry VIII is published by Robinson. Peter the Great is published by Hutchinson. Derek Wilson is an award-winning historian and author of the highly acclaimed biographies of Hans Holbein and Sir Francis Walsingham, and The Uncrowned Kings of England (about the Dudleys). He runs the annual Cambridge History Festival and is available for talks. To find out more visit: www.derekwilson.com.