John Smith was at once a visionary and a man of action, and over more than half a century proved to be the country’s most resourceful, effective and successful champion of preservation. A man who in his own phrase loved “burning decks”, he was the first to come comprehensively to the rescue of what the French call the petit patrimoine, minor but delightful buildings such as lighthouses, Martello towers, garden temples and follies, gatehouses, banqueting halls, mills, quirky cottages and even a Doric pigsty. Smith also had a sustained interest in military buildings, artillery forts, gun batteries and dockyards, which he saw as superb essays of construction.

The spark for the Landmark Trust came from Smith’s experience in advising the National Trust, which, he realised, could take on only buildings with endowments. Smith intended his charges to earn their keep. He explained his philosophy in the first Landmark Handbook “by living there, however briefly, people get far more out of a place than just by looking at it; they can study it at leisure, be there early and late, in all weathers and lights, and get the feel of its surroundings”.

The Landmark Trust was very much a joint creation with his wife Christian and in the early years an extension of his household. She supervised the furnishing of the Landmarks and curtains and soft furnishings were made in the home farm at Shottesbrooke, near Maidenhead, the house of the Vansittarts, which he inherited from cousins and reduced in size to make it more manageable. One of his favourite sayings was: “Buildings are seldom improved by additions but very often by subtraction.”

Smith’s genius lay in setting up the Manifold Trust to make the money and the Landmark to spend it. He made money for Manifold by buying up the tail end of Belgravia leases, which few would touch because of the potentially large reparations due to the landlord when the lease fell in. These properties could be let very lucratively to diplomats and businessmen, but Smith cautioned that “you could lose as much on one as you could make on 30.”

The money made by Manifold enabled Smith to espouse pioneering causes on an unprecedented scale. Through his friendship with L. T. C. Rolt, father of industrial archaeology, Smith became involved with canal restoration. He bought a stretch of the Stratford-upon-Avon canal running through the park at Charlecote, that he persuaded the National Trust to take on, and Landmark also took on several lock-keeper’s cottages in its early years.

Smith also made a surpassing contribution to the preservation of historic ships — notably Brunel’s SS Great Britain, returned to Bristol from the Falkland Islands in 1970, and then to the wartime cruiser HMS Belfast, moved to her permanent berth on the Thames in 1971. Most important of all was HMS Warrior, Britain’s only preserved Victorian battleship. Napoleon III described the 1860 iron-hulled, steamship as “a snake among the rabbits”, outgunning and outrunning the opposition by such a margin that it never had to fire a shot in anger.

Since the 1920s Warrior had been a floating jetty at Pembroke Dock. In 1979, when the oil depot closed, the Ministry of Defence gave her to the Warrior Preservation Trust (set up by Smith for the purpose). He then contributed £10 million to a comprehensive restoration of this ship, correct in every detail. His intention, he said, was that it should appear to the visitor like the Marie Celeste, as if the entire crew had gone ashore a few minutes before.

Another imaginative gesture was to buy Captain Oates’s medals at auction and give them to his regiment, the Royal Iniskilling Dragoon Guards.

Having bought a house at 21 Dean’s Yard, Westminster, Smith was rapidly drawn into the life of the Abbey. When he heard that Westminster School was no longer staging the traditional Greek play because the magnificent backcloth — a panorama of ancient Athens — had rotted, Smith paid £5,000 to have it restored.

He was a great traveller and prided himself on being the first man to visit all the explorers’ huts in Antarctica.

Smith’s financial backing kicked off the rescue of Barlaston Hall at a time when the National Coal Board was refusing to pay compensation for mining subsidence damage.

Incensed by the treatment of historic buildings on Gibraltar, he financed a list of 1,100 historic buildings that the Government should protect (it has never done so). He also sent a £25,000 donation (the largest) to start a museum at St Helena.

Smith regularly clashed with officials who frustrated his restoration plans — English Heritage was portrayed as a sinking ship “holed” by a broadside from Warrioron the plate commemorating the 25th anniversary of the Landmark Trust.

Always dapper, Smith was impatient and demanding. His astuteness ensured that he was much sought after for committees but he rarely remained long, doing two years at the Redundant Churches Fund, two at the National Heritage Memorial Fund and three as Lord Lieutenant of Berkshire. Similarly, his tenure as MP for the Cities of London and Westminster lasted just five years.

John Lindsay Eric Smith was born in London in 1923 and grew up in Sussex. His parents had reduced their sprawling Victorian house, Ashfold, from 40 bedrooms to 24. The works there gave the young John his first taste of a building site. He used to frequent the estate carpenters’ shop and became fascinated by tools and materials.

The Smiths were the oldest and grandest of banking families. Their firm had emerged in Nottingham in 1658, even before the Hoares. After the First World War the firm merged with the National Provincial Bank. John Smith’s father, Eric, became chairman of this merged bank, and Coutts was one of its subsidiary businesses. John Smith thus became the ninth generation, in unbroken succession from father to son, to work in the Smith bank or its successors.

After Eton he joined the Fleet Air Arm and was a navigator in the dive-bombing raid on the battleship Tirpitz in Kvaenangen fjord, Norway, in July 1944 (his Barracuda was holed by flak and ran out of fuel as it landed on its carrier). He was serving in Ceylon when the war ended.

At 22 he went to Oxford, a city for which he never lost his affection. There began his lifelong friendship with Teddy Hall, the father of carbon dating. In 1950 Smith became a director of Coutts, turning his back on his long-held ambition of becoming an architect. Yet he remained close to several of the finest but more traditionally minded architects of his day, including Raymond Erith, Francis Pollen and Philip Jebb.

Appointed CBE in 1975, Smith was knighted in 1988.

A favourite phrase of his was: “I mustn’t bore you but . . . ”, always the prelude to an interesting anecdote or unusual fact. In recent years he had devoted much time to his diaries — it is not yet known whether they will be published.

Smith is survived by his wife and their two sons and two daughters; a third daughter predeceased him.

Sir John Smith, financier and philanthropist, was born on April 3, 1923. He died on February 28, 2007, aged 83