I write the Sharpe books that made Sean Bean famous - my new 1 may be last ever

Since Sharpe's Eagle introduced the "tall, black-haired lieutenant with the slung rifle and the scar" in 1981, Bernard Cornwell's bestselling Napoleonic Wars-era novels have invariably ended with the promise of further adventures. The latest in the mind-blowingly popular series, brought to life on TV starring Sean Bean as the roguish rifleman, ends slightly more equivocally however. Rather than promising in his afterword that Richard Sharpe and Sergeant Patrick Harper will "march again" as has been his custom, Cornwell writes: "I hope they do, but can make no promises."

Visiting London to launch his new book, Sharpe's Storm, the 24th in the series, of which more shortly, the author wryly admits he's running out of stories and, possibly, time. "I suspect that is the last, although who knows," he tells me.

"If I'm still alive, I might decide to fill in another gap. I haven't taken him to Flanders yet, which would be a very young Private Sharpe pre-India. But he's been with me for almost 50 years and he's in my head. When I'm walking the dog, I occasionally hear Sharpe. So even if I don't write another one, I'm sure he'll accompany me to the end."

Fans can take solace in the fact Cornwell remains a hale and hearty 81-year-old and I wouldn't bet against him writing more Sharpe when the urge takes him. Indeed, he adds with a chuckle: "I can't say that three years from now I won't suddenly think, 'Oh, God, let's let the bugger loose again'."

We can only hope but, at present, post-Sharpe's Storm, set in the winter of 1813 and featuring the Battle of St Pierre, the rifleman who came up from the ranks, saved the future Duke of Wellington's life to earn a battlefield commission, and proved an ongoing thorn in the side of the French appears all but retired from soldiering.

Happily, Cornwell does reveal he is working on another chapter in his bestselling Last Kingdom series, set during the great viking invasions and recounting the creation of England by Alfred the Great and his descendents through the eyes of Cornwell's distant ancestor Uhtred of Bebbanburg.

"I'm not going to tell you much but it takes Uhtred abroad," he says. "He's been to Frisia, Iceland and Denmark. This one is going to take him across to Francia [modern-day France] - but that's all I know, because I haven't got beyond the fourth chapter!"

The forthcoming book will slot into the existing 13-novel series, televised by Netflix and starring Alexander Dreymon as Uhtred. "I'm rewriting something in the middle which never gets mentioned again because all the other subsequent books have been written," admits Cornwell with a smile.

"It's a rather large occasion in his life, but I keep thinking, 'Oh well...' It's like Sharpe never mentions to anybody that he was at the Battle of Trafalgar because that book was written much later than many of its chronological successors!"

Such chronological complications have been a common occurrence for Cornwell, who started his first series in 1809 with Sharpe's Eagle, then, after several books, dipped back in time to set prequel Sharpe's Tiger in 1799. "If I was sensible, I would have begun at the beginning and kept going," he says.

"In the early books, I blithely mentioned Sharpe's career in India, thinking, 'I'll never write those books so it doesn't matter what I say'. I claim he learned to read in the dungeons of Seringapatam and then, when I finally came to write Sharpe's Tiger, I thought, 'F**k, he can't have been in these dungeons more than four or five days at the most!'"

How then does he keep up? "I have a spreadsheet which I paid a young student to make for me of every character who appears in the first Sharpe series and their fate - so that I don't have dead soldiers coming back to life, although I'm sure I do," he chuckles. Many fans, Cornwall admits, know the characters and plots far better than him.

As if to prove it, he admits to never having re-read Sharpe's Eagle - the debut book that made his name when, as a young TV producer, he fell in love with an American, Judy, now his wife, and moved to the US without a green card so unable to work. Having long been "fanatical" about the Napoleonic Wars, and having failed to find a series that did for Wellington's Army what CS Forester's brilliant Horatio Hornblower saga had done for the Royal Navy during that period, he decided to write it himself.

Written in six months and published in 1981, Sharpe's Eagle it was swiftly followed by Sharpe's Gold and Sharpe's Company. Between them, they changed his fortunes from out-of-work TV producer to internationally bestselling historical novelist.

"I always knew that, if I was going to want to write novels, it would be Hornblower on land," he adds. "When George Lucas started Star Wars, he sold that to the network in America by saying it was his Hornblower in outer space!"

If it was a gamble, it was one that paid off and handsomely. Some 55 books have sold more than 20 million copies around the world - inspiring two huge TV adaptations, making Sean Bean a superstar, and delighting millions of fans worldwide.

Today, ranked alongside his hero Forester, as well as other writers of historical fiction such as Patrick O'Brian (Jack Aubry) and George MacDonald Fraser (Flashman), he's a citizen of the US. "I think it was 2000, so 25 years ago, by which time I knew I was almost certainly going to live and die in the States." he recalls. "I was paying enough tax, and I thought they had a point, 'No taxation, without representation', and I wanted to vote."

How funny, I point out, that one of our most quintessentially English authors, whose books have shone a light into some criminally disregarded parts of our island nation's great history, should have spent his entire career writing from abroad.

"I think it makes it easier in a way," says Cornwell. "I tend to be a bit rosy-eyed about England. I'm very aware that I'm a very British author, but then so was PG Wodehouse and so was CS Forrester and, like me, both of them wrote in America."

He doesn't rule out moving back to the UK at some point - "the cricket's better," he says - but, for now, home is split between Charleston, South Carolina, and Cape Cod in Massachusetts.

The Duke of Wellington's Army in the Peninsular War - which involved England, Spain and Portugal fighting against France for control of the Iberian Peninsula - has sometimes been described as "gutterborn scum commanded by aristocrats and disciplined by brutality". Wellington himself wrote in July 1813: "We have in the service the scum of the earth as common soldiers." While Cornwell concedes that many joined the Army in desperation, he disputes that description.

"Wellington described it as the best army he could imagine and a lot of historians would say it was probably the best Army Britain ever sent abroad," he says. "Most of the officers were what we'd call middle-class, the sons of clergymen and merchants, and there was an enormous amount of affection between the men and the officers which I tried to reflect in some of the books.

"Yes, discipline was strict. Not every officer was a flogger, and some officers disapproved of it while others loved it, but it was plainly an army of high morale, high discipline and enormous effectiveness.

"People always quote the 'scum of the earth' but, in 1812, Wellington wrote a letter to Lord Bathurst, who was the Minister of War here in London, and described them as the 'finest, bravest troops in the world'. And they never let him down. I think I quote Wellington in this book where he talks of [French commander] Marshal Soult: 'When he gets into a difficulty, his troops don't get him out of it; mine always do'. He was enormously proud of them."

Cornwell believes too many authors see a TV adaptation as the be-all and end-all, which he considers a mistake, but concedes the Sharpe series was very good to him. "I was vaguely relieved I had a number one bestseller before the series. The Last Kingdom didn't hurt the Uhtred books either.

"I often meet people who only started reading them because of the TV series. Not that I'm involved in them, although I did get a cameo appearance in The Last Kingdom." He played a viking who is killed by his own hero, which seems somehow Freudian. "Ungrateful sod," chortles Cornwell.

Sharpe's Storm features a feat of military engineering considered extraordinary for the time - the building of a huge pontoon bridge across the River Adour in south-west France so Wellington's troops could march on Bordeaux. It was due to be a huge set piece.

"And then I realised, literally when I was halfway through the book, that I'd already said Sharpe was somewhere else when that was built," says Cornwell. "I thought, 'F**k, I haven't got my ending', so I invented the ending instead - which I rather liked. I thought, 'Well somebody must have done a reconnaissance of that northern bank, but it's not in any of the books'.

So Sharpe, Harper and the rifles, as ever, are dispatched to do their best. It's a typically thrilling tale, told with Cornwell's usual panache and brilliance.

Before he leaves me to try some Rifleman's Pale Ale, specially created by The Kirkstall Brewery in honour of his most famous character, Cornwell pays tribute to his long-time editor, Susan Watt, who died last year aged 86 having edited every single one of his Sharpe books and may of his others too.

All too often, editors are the unsung heroes, but as he says: "She was a frightfully smart, frightfully kind woman and we became very close friends. She felt very maternal about Sharpe. She was always wondering what he was feeling. I said, 'Susan, he has two moods, grumpy and angry'. But she was a wonderful editor, her death was a horrible surprise."

For his own part, I wonder if Sharpe has given him an element of immortality? "Oh God, I hope not. I always warn my wife: 'When I die, the sales are going to go like that'." He points downwards. It seems unlikely.

Recently, Cornwell has been thinking of doing another book featuring Thomas of Hookton, an archer whose adventures take place in the early days of The 100 Years War against the backdrop of the 14th century search for the Holy Grail.

"I re-read the three of them, Heretic, Vagabond and Harlequin, and got depressed," he adds. "I thought, 'F**k, I used to be good'." Whisper it, but he still is.

• Sharpe's Storm by Bernard Cornwell (HarperCollins, £22) is out now