There is no doubt that Arthur Wellesley, the Duke of Wellington, was in command of the allied army during that gruelingly hellish day. But at that time, he had yet to be dubbed the "Iron Duke." In fact, that epithet was given to him many years later, long after the Regency was over.
There are a number different stories about how the Iron Duke acquired his sobriquet. In addition, there were other noble military men who had been called Iron Duke, long before Wellington was born.
The most commonly told story of how the Duke of Wellington became known as the Iron Duke was that it came to him in a roundabout way from a mail steamer. According to this story, in the early 1840s, an iron steamship, a novelty at the time, was built to ply the Dublin to Liverpool route. When it was launched in the Mersey, its owners christened it the Duke of Wellington. It was a sturdy and reliable ship, and soon began to be called the "Iron Duke." In 1844, this ship was involved in a collision with the brig Parana, and came away with very little damage. When the story hit the newspapers, this same the ship’s nickname was applied to the Duke himself, at first in jest. But soon it was used in ernest, as he was once again in favor with the populace, as an honorable man of great courage and determination.
Another story suggests that Wellington began to be called the Iron Duke when, after the rejection of the Second Reform Act, in 1831, he made some special renovations to his London home, Apsley House. When the Second Reform Act was debated and later defeated by the House of Lords, mob violence erupted in a number of places across Britain, including London. One of the targets of that violence were the windows of Apsley House, a number of which were smashed by a mob. This first occurred on 27 April 1831, only three days after the unexpected death of the Duchess, after a short illness. His wife’s body was still in the house, being prepared for burial, as the window glass was shattered. The mob attacked again, on 12 October 1831, smashing more windows. This time, the Duke was nearly hit by one stone as he sat writing at his desk. That same stone also damaged one of his paintings, hanging on the other side of the room. Wellington was a supremely practical man, and saw no reason to have his windows damaged again. Therefore, he had louvered, bullet- and rock-proof iron shutters, sometimes described as Venetian blinds, fitted to the windows of Apsley House.
Those iron shutters remained at Apsley House for the rest of Wellington’s life. Almost twenty years later, a friend asked him if it was not time to take the iron shutters down. The Duke only laughed. "They shall stay where they are," he said, "as a monument of the gullibility of a mob, and the worthlessness of that sort of popularity for which they who give it can assign no good reason. I don’t blame the men that broke my windows. They only did what they were instigated to do by others who ought to have known better. But if any one is disposed to grow giddy with popular applause, I think a glance towards these iron shutters will soon sober him." The iron shutters remained in place until they were ordered removed by Wellington’s son, the second duke, many years after his father’s passing.
Yet another story of the origin of the epithet "the Iron Duke" as applied to the Duke of Wellington, suggests that it was because he was such a stern disciplinarian when dealing with his troops. However, there is very little evidence to support this theory, Though they were known to refer to him as Old Nosey, and they often did not like the restrictions he put on their more riotous conduct, his troops held him in great admiration and affection, and routinely cheered his appearance on the battlefield.
Wellington’s personal behavior has also been suggested as a possible source of the sobriquet. He was known to be a man of strong moral fiber, loyal to King and Crown, and though rather soft-hearted, he kept his emotions firmly in check whenever he was in public. He was known to have an amazingly steady nerve and was unswerving in his adherence to principles. As a statesman, he was a dedicated and responsible field commander, diplomat, member of Parliament, Commander-In-Chief and Prime Minister.
I have found references to a poem, an "Ode to Waterloo," written by "a poet of no inconsiderable reputation" which was published in 1845. These references are vague, with no exact title or author’s name given, and I have not been able to find the actual poem. However, according to sources I have found, this poem is the first to refer to Wellington as "the Iron Duke," and it was intended "as a felicitous expression of his general character."
I have yet to find evidence that Wellington was referred to as the Iron Duke in print before the mid-1840s. This would appear to rule out the installation of the iron shutters at Apsley House as the source of the epithet, since that had occurred more than ten years previously. In 1847, the Great Western Railway built a locomotive at its Swindon locomotive works, which they dubbed the Iron Duke. This is particularly ironic, as Wellington very much disliked trail travel. But the company would not have named their new locomotive after an unpopular public figure. I believe that the news reports of the maritime accident between the Parana and the Iron Duke first brought the sobriquet to the attention of the public. At that time Wellington was again popular with the public and the nickname was adopted out of affection and respect for the hero of Waterloo and a venerated elder statesman.
Almost two centuries before the birth of Arthur Wellesley, two different European nobles had been called Iron Duke. The first was Fernando Álvarez de Toledo, 3rd Duke of Alba, a Spanish general who later became governor of the Spanish Netherlands. His cruelty to the Protestants there earned him the pejorative designation of Iron Duke. Ironically, a score of years later, a Huguenot nobleman and military engineer became advisor to King Henry IV of France. Maximilien de Béthune, duc de Sully became known as the Iron Duke when he drastically reformed the abuses rampant in court bureaucracies, particularly reorganizing government finances. This Iron Duke was hated by the Catholics because he was a Protestant, and hated by the Protestants because he was loyal to a Catholic King. The Duke of Wellington was the first noble military man to be called Iron Duke with respect and affection.
During the Regency, the Duke of Wellington was called "The Beau" by many of his officers and friends, partly because he was always well dressed, and partly because he was very popular with the ladies. His troops were known to refer to him as "Old Nosey" or "Old Hookey," because of his pronounced aquiline nose. There is no doubt that Wellington was called the Iron Duke, though it is not clear exactly how he acquired that sobriquet. But it is clear that he was never referred to in that way during the years of the Regency. In fact, it appears he only began to be called the Iron Duke well into the reign of the Regent’s niece, Queen Victoria. Therefore, it is historically inaccurate for an author to use that nickname for the great general in a novel set in the Regency.