One of Britain's most gifted actors, Robert Hardy is known to millions for such portrayals as Winston Churchill in The Wilderness Years, Siegfried Farnon in All Creatures Great and Small, and Cornelius Fudge, minister of magic in the Harry Potter films.

To those who know only his popular screen roles, however, it may come as a surprise to learn that he is also a scholarly and highly regarded expert on the longbow. Robert Hardy was a trustee for 11 years of the Royal Armouries at HM Tower of London, and remains a trustee and consultant to the Mary Rose Trust. Originally published 30 years ago, his Longbow, a Social and Military History is a key study and this year comes out in paperback for the first time.

It is a wonderfully readable book, richly illustrated and full of fascinating insights.

It has never been out of print for 30 years,' he told me when we met at his Cotswold home. "My publishers can't believe it and nor can I because it is such a narrow focus book, you know."

Born in 1925, Robert Hardy read English under Tolkien and CS Lewis at Magdalen College, Oxford. He told me of his first tutorial with Tolkien at the Inklings' favoured pub on St Giles, The Eagle and Child (or The Bird and Baby' as it was known). Tolkien placed six pints on the table and, covering his eyes, asked the undergraduates to speak for a minute or so, in turn.

By their accents, he then identified with complete precision what part of the country they came from. Robert Hardy, he said, was evidently from the Welsh borders - Shropshire, Montgomeryshire - but spoke with a regrettable London influence'.

"He was absolutely right," Robert laughed.

It has often been reported that he developed his interest in mediaeval warfare while appearing as Henry V in a stage production of Shakespeare's play.

But in conversation it became clear that his enthusiasm predated his acting career. He had played bows and arrows during his Shropshire childhood: "I used something I cut out of the hedge, with a piece of string and a reasonably straight twig. Luckily I never did any damage, but in the village a boy's eye was shot out with just such a hedgerow bow. It is quite lethal.

"I was about six or seven when, at home in the attic, I found two longbows. But at that time I was too weak to do anything with them. I should think they were late 18th century and they were dry as bones. When I was strong enough to put a string on them and try and bend them I broke both.

"There wasn't any archery at school - at least not that I came across - so there was a long gap when I did not touch a bow. Then in the war I went out and trained in Texas. I was in a very small group who were selected as putative fighter pilots. It was unbelievable. I mean, I came out of England with its wartime privations and there was sugar, chocolate, cream, butter - ridiculous.

"One of the things they did in training was to put longbows into our hands, in order to teach us aiming off and allowing for windage and that sort of thing. They were the American version, which is a sort of flat longbow. It is a very efficient weapon and I took to it like a duck to water. I kept on going back to ask if I could do some more and they said, No - you do aircraft recognition!'"

From that time his interest gradually deepened. He would become a member of the British Long-Bow Society (formed in 1951), and while pursuing his acting career, spent ten years on research before publishing his book on the longbow in 1976.

Three years later, dramatic discoveries at a wreck site in the Solent would call for a new edition. One day in 1979 a longbow was brought up from hull of Henry VIII's great ship the Mary Rose, sunk during battle with the invading French fleet in 1545. For some years the patient work of exploring the wreck had been going on under the inspirational direction of the archaeologist, Dr Margaret Rule.

"Her desk was full of volumes on cordage rope-making, chain-making, cable-making, mast-making and ship-building of the Tudor period. She had to get abreast of it because they were scouring the ship before they brought her up, and she needed to identify things while she was swimming underneath. So she really studied like a fiend and among the books she read was my 1976 book.

"One day the chief diver came up bearing a great long pole and Margaret said, Good God, it's a longbow'. She reckoned it to be one of the 240 which are mentioned in the loading lists of the Mary Rose, in the Anthony Roll, at the Pepys Library in Magdalene College, Cambridge.

"It is a catalogue of all the king's ships of the period and what they carried: how many men, how many guns, how many bows etc. When she said it was a longbow somebody leant over her shoulder and said, "I think it's just a pikestaff, you know, it's very big". And she said, "Well now, wait a minute, there's a book". She pulled out my volume and said, "This man, Robert Hardy, get hold of him."' The telephone rang and I was absolutely beside myself with excitement.' Almost no longbows of true antiquity had ever been authenticated before. From the dark, silted bed of the Solent a total of 138 would now see the light - an extraordinary cache. They were, in general, in miraculously good condition, and as the bows were brought to the workrooms of the Mary Rose Trust, Robert Hardy was among the team of three invited to conserve and examine the weapons. His colleagues were Peter Pratt, a professor of crystal physics and expert on tension and the behaviour of material under stress. The third member of our dotty little group was the only professor of wood science, I think, that has existed. I don't think he was replaced when he retired. John Levy was his name, from the Imperial College of Science and Technology, and we got together because we had all met at a symposium at Reading University back in early 70s and we objected to the way people were underestimating the skills of our ancestors. There was a general feeling that our forebears back in the 13th-14th centuries were covered with hair and trailed their knuckles on the ground'.

The team's meticulous researches helped to reveal how much expertise had gone into the evolution of the yew-wood bow that was England's all-conquering war weapon in the age of Crcy and Agincourt. The devastating instrument allowed archers to loose ten arrows a minute from distances of as much as 300 yards - and still penetrate metal armour. The blizzard of arrows at Agincourt left 10,000 French dead, fallen on the field or massacred soon afterwards; incredibly English losses were a few hundred at most.

Before reading Robert Hardy's book I had myself naively imagined that to make their great war bows, the English craftsmen simply selected sturdy branches of yew from the nearest greenwood. In fact they selected wood from the trunk and carpentered it with extraordinary care, from the boundary between the sapwood and the heartwood. The tree had to be sizeable and straight-grown, at least 100 years old. From one log only four or five bows might be taken.

You cut in like a small slice of cake - a blunt-ended slice of cake. On the outside is the sapwood which is the best known timber of all in the world for resisting tension. And lying next to it is the heartwood which is different colour altogether. The sap is a sort of lemony colour and the heart is brown to dark brown, and that is the best known timber of all to resist compression. So you get a god-given lamination in the tree.' The yew had to have other attributes. The best trees did not come from the English greenwood at all. They came from Europe: from forests in places like Spain, Austria, Italy and Poland where harsher summers and winters caused trees to grow slowly and fight for their survival, developing a special resilience. You won't believe this but it's now absolutely proven that we imported so many millions of staves that the yew stands are only now beginning to recover all these hundreds of years later.' As for the archers, the Mary Rose team proved incontrovertibly that the English longbow had a mighty 100-180 pounds draw weight, requiring men of exceptional strength to pull the drawstrings. I said in the first edition that in my view the longbow used at battles like Crcy, Poitiers and Agincourt - my instinct told me that to be effective it must have a strength of something like 140-150 pounds draw weight. I discovered that there was a famous American hunter called Howard Hill who hunted big game in America and he went to Africa and shot elephant with a longbow. His favourite hunting bow was 172 pounds draw weight, which is beyond most people's capability.' Many experts had regarded such weights as implausible. But research into the draw weights of the Mary Rose bows proved Hardy right. Moreover, skeletons with archery tackle found in the hull were of hefty men; not necessarily tall but massively boned.

Only through long practice, it appears, could use of the formidable bows be mastered. And practice was an obligation for all Englishmen. In royal proclamation after royal proclamation, reign after reign it was decreed that on a Sabbath or feast day when you were free of your labours you were to go and practise at the butts which were behind every church in every village in the land.' Scratch marks on the masonry of many parish churches show where men would sharpen their arrows. They're generally at the base of stone pillars where they could get at it easily. Usually in the porch, or the lych where they could sit and quietly work away.' Robert Hardy spoke fondly of A.J. Stirland, the pathological consultant to the Mary Rose, whose book Raising the Dead used archaeological and skeletal evidence to give insight into the men that served upon ship; from their age and height to their diet and physical condition. The work involved detailed study of the macabre remains, some of which showed distortions in the shoulder blades that may have resulted from working with heavy bows. She was an extraordinary woman, a lovely woman. I said you live right up in Norfolk - do you go down to Portsmouth every day and she said, "No, no, no. I've got over thirty of the crew in my spare bedroom"'.

The longbow remains a passion with Robert Hardy. He continues to give talks on the subject and keeps abreast all the latest research. This year marks the 40th anniversary of the first ever dive on the Mary Rose wreck site; and his excitement at the ship's revelations remains undimmed. As he writes in Longbow, The lifting of that first bow, through the surface of the Solent, was to me as magical as the appearance of Excalibur above the mere.' Longbow - A Social and Military History £14.99 is available in paperback from Sutton Publishing.

Robert Hardy is also author (with Matthew Strickland) of The Great Warbow.