On the Aberdeen Peninsula, at the westernmost point of the little African country of Sierra Leone, there stands an elegant lighthouse. It was built by the Portuguese more than 200 years ago to guide ships from the Atlantic into Freetown, the best deep-water harbour for 1,000 miles or more. The haven is the reason that a British colony of freed and rescued slaves was founded here in 1792. But now the plastered walls of the structure are crumbling and pieces of crucial ironwork have fallen to the ground.

The lighthouse keeper, Yamba Banday, told me he couldn't get up the collapsed stairs to service the workings of the light. The generator and its back-up had failed and so the light was now connected to the city power supply – which frequently gives out – by a simple domestic cable. In fact, Banday agreed sorrowfully, the light was less reliable now than at any time since the Portuguese built it. "This is the sad truth about our country," he said. "Everywhere you look, there's always more to be done."

But things had improved a lot, he agreed, since we had last met. That was 10 years ago, when Sierra Leone was in the grip of a notoriously savage civil war and the capital itself was under threat. Banday remembered the sudden arrival in May 2000 of a British force – some had come ashore on the beach opposite the lighthouse – of paratroops and Royal Marines. Under David Richards (then a brigadier, now chief of the general staff), Operation Palliser was first billed as a simple evacuation mission for expatriates. But that was a nervous Downing Street cover story. The real task was to bolster a UN operation that was on the point of losing control of Freetown to the vicious militias that controlled most of the country and had taken hundreds of peacekeepers hostage.

There was no strategic or commercial interest in the adventure, and none among the British public; this was the Blair/Cook "foreign policy with an ethical dimension" in its purest, most altruistic form. But the Sierra Leone intervention worked – uniquely well, in the history of modern military interventions in Africa.

The rebel forces were scared away from the city, the UN got off its knees and the government army was revitalised. Eighteen months later, Sierra Leone's 11-year civil war was brought to an end. In the streets of Freetown at the time the graffiti read: "Queen Elizabeth for king!" and "Return to us our colonial mother!" Tony Blair remains more popular here than anywhere else on the planet. He still visits the country every couple of years, and officials from his office are seconded into the finance and health ministries. Several Sierra Leoneans said they would personally campaign for Blair to be the country's president. A young Freetown documentary-maker, Arthur Pratt, told me: "We think we are to him as a favourite child."

Robin Cook promised that Britain would "rebuild Sierra Leone". Today it receives more British aid money per capita than any other country in Africa. Has this aid done any good? On most conventional indicators, Sierra Leone is still what it was 10 years ago – one of Africa's poorest countries, with the worst maternal mortality rates in the world. One in eight Sierra Leonean women dies in childbirth.

But 10 years on we drove to the diamond-mining district of Kenema, source of much of Sierra Leone's troubles, on an immaculate new road built by the Chinese. It was almost empty of cars, but on the verges a new business had sprung up – old ladies selling piles of rocks they had gathered from their fields to passing construction workers.

In 2000, Freetown's Aberdeen Peninsula was the base of UN peacekeeping operations and the journalists' hotel. Now the war-ruined buildings had been turned into red-lantern-hung hotels and casinos with Chinese names. Even the hotel gardeners were Chinese – the Sierra Leonean gate guard at the Bintumani Hotel told me that the only jobs available for Africans there were security work. The prostitutes in the casinos and the nearby beaches looked less bedraggled than when they depended on UN peacekeepers for a living. There were the beginnings of an adventure tourism trade.

There is now a new attempt, organised by the Department for International Development (Dfid) with the Sierra Leonean government, to address the problem of basic health services in a country of six million that has 95 midwives and six obstetricians.

I visited some of the health clinics in Freetown that British taxpayers' money is supporting. It took me to places such as the slums of Dwarzak Farm that were among the scariest in Africa 10 years ago. On the compound wall of a clinic in George Brook, the graffiti announced this was "West Side Niggaz" territory. These were one of Sierra Leone's chief terrors: teenage gunmen in big shorts, styled straight from hip-hop videos. They raped and killed, and assisted in the wholesale amputation of hands and arms.

The West Side Boys' (the British media felt obliged to rename them) greatest moment was the kidnapping in August 2000 of 11 soldiers of the Royal Irish Regiment who were training recruits for the government army. A bloody rescue by the SAS left most of the West Siders dead, as well as one British soldier. But that turned out to be the last significant event in Sierra Leone's long civil war. The graffiti are long faded and people laughed when I asked about it. "That is history," said one shopkeeper.

There were successful elections in 2007 and the bullet holes that pocked the grand colonial buildings of downtown Freetown have been plastered over. Nevertheless, as Arthur Pratt says: "The youth unemployment, the desperate inequalities, are still present." Much depends on stability in Liberia, whose own vicious wars have always spilled over into Sierra Leone. At least the surface diamond fields of Kenema and Kono, which financed rebel movements and encouraged the interference of Liberian dictators, are now exhausted.

I was told, again and again, that the country now knows what war means. "Everybody smelled the war, everyone felt it," said Ibrahim Moseray, the dynamic organiser of the Fair Trade cocoa co-op in Kenema (it supplies British chocolate company Divine).

Yamba Banday, ever-confident, said he had high hopes that USAid would come up with some funds to repair the lighthouse. And he asked me to give his best wishes to Tony Blair.