In the oil-rich state of Oman, he was known as the White Sultan, after helping to stage the British-backed coup that brought its leader, Qaboos bin Said al-Said, to power in 1970. Back home, however, Brigadier Tim Landon, one of Britain’s richest and most secretive men, lived the life of a virtual recluse.

Over the decades since, Landon – rumoured to have received a cheque for £1m from Qaboos each year on his birthday – amassed a property portfolio of 50,000 acres in England. Within it were two grouse moors in North Yorkshire, totalling 11,818 acres, which go on sale this week for £15m, following Landon’s death in July, at the age of 64. This is the largest single chunk of prime sporting countryside to go on the open market in Britain for a decade.

Landon reputedly met Qaboos when they were both at Sandhurst, and became a confidant of the Omani ruler after the bloodless coup that helped to transform the sultanate into a modern country. He helped Qaboos see off a rebellion by Marxist insurgents and acted as a conduit for trade with the west. He went on to amass a fortune estimated at £500m by The Sunday Times Rich List earlier this year. Only one grainy black-and-white photograph of Landon appears ever to have been published. Dating from the time of the coup, the picture shows the Canadian-born businessman sitting in a Land Rover, clad in army fatigues and a turban, surrounded by gun-toting members of the Omani army.

Ranulph Fiennes, the explorer, who was in the SAS, spent two years with Landon in Oman. He recalled someone who got on with the locals and learnt their language. “We’d never had good intelligence,” he said. “As soon as Tim came, we started getting the right information. We started killing people.”

As well as spending his money on grouse moors, the ultimate rich man’s pursuit, Landon indulged his other passion – motor yachts. One of them, valued at £17.4m, was reckoned at the time to be the biggest and most expensive private yacht of its class ever built. He named it Katalina, after his wife, a descendant of the Austro-Hungarian nobility.

The two adjoining moors on the market are the Westerdale Estate, priced at £8m, and the Rosedale Estate, on sale for £7m. They lie in a national park, 45 minutes from Teesside airport and York. As well as the expanses of blazing purple-heather moor, woodland and farmland, the two estates include a 12-bedroom hunting lodge, nine cottages, a hotel, eight farms, six farmhouses, a cricket pitch and a lunch hut. The sellers would like the two moors to go to one buyer, but would be prepared to split them.

Landon paid £6.5m for the land in 1995. He bought it for his son, Arthur, who, according to newspaper reports at the time, was to start at Ampleforth, the Roman Catholic public school in Yorkshire. “We thought it would be nice to buy a grouse moor for Arthur so he can go shooting at weekends,” his mother was quoted as saying.

The reason for the sale? “The owners are restructuring their portfolio,” says Clive Hopkins, head of farms and estates at Knight Frank, the joint selling agent. “They are the finest moors in North Yorkshire, and the most diverse from a sporting and residential point of view.”

It is the first time for years that a significant moor has come to the market without Landon bidding for it. He bought, then sold on, Knarsdale, in Northumberland. In 1994, he lost out on Gunnerside, a 36,000-acre estate in North Yorkshire that sold for £10m.

Shooting estates can be financially perilous. Grouse cannot be bred in captivity, so owners have little control over the success – and, therefore, value – of their shoot. A fickle, fluctuating game population can drive down the value considerably. Owners must invest heavily in conservation and good staff. Landon did both.

Qaboos’s father, the deposed sultan, Said Taimur, reportedly declared, as he lay dying in a suite on the top floor of the Dorchester in 1972, that his only regret was “not having Landon shot”. In the event, Landon died of lung cancer and was buried at St Barnabas, the church at Faccombe, a village he owned in Hampshire, along with 4,000 acres of surrounding countryside.

The Westerdale Estate and the Rosedale Estate are for sale jointly for £15m with Knight Frank (020 7629 8171, www.knightfrank.co.uk ) and CKD Kennedy Macpherson (020 7409 1944, www.ckd.co.uk )

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