As Friday prayers end thousands of believers stream out of the mosque. Keen-eyed men with Pashtun cheekbones hover by a convoy of cars, many of them wearing the traditional hats of Pakistan’s North-West Frontier province. Were it not for the Tube depot in the background, one could be in that exotic land with its long and sometimes unhappy links with Britain. Departing last, and driven safely away, is Hadhrat Mizra Masroor Ahmad, the fifth leader of the worldwide Ahmadiyya community.

Built on the site of a former dairy in 2003, the Baitul Futuh in Morden, south London, is the largest mosque in western Europe, with capacity for 10,000 people. When I first visit, around 6,000 believers are packed into the complex, bowed in prayer. There is an airport-style X-ray machine by the entrances, something churchgoers might find alarming, although the Baitul Futuh looks lightly protected compared to the Ahmadiyya mosque in nearby Putney.

There was some local hostility when the mosque opened. Religious centres are rarely welcomed by any community, bringing as they do not just traffic and noise but inevitably more believers to the area, and in a neighbourhood inhabited by many people who fled inner London a generation ago there was bound to be difficulty. Yet the Ahmadiyya are nothing if not good neighbours. The mosque’s conference hall is used by various local civic bodies, and every year chocolate is handed out to local people at Eid, a small gesture that nonetheless warms relations. Besides which, the security is not to protect them from BNP supporters.

Ahmadiyya is an Islamic reformist movement that began more than a century ago in British India. Little known outside Pakistan, where half of its ten million followers live, it has nonetheless attracted some attention in Britain for its now annual poppy-selling drive, which raises more than £20,000 a year for the British Legion.

The Ahmadi follow Mirza Ghulam Ahmad, born in the Punjab in 1835, who in 1889 declared himself to be the promised messiah (Imam Mahdi). After his death in 1908 his followers split, with some taking the orthodox Sunni view that there could be no more prophets, and the others, the Ahmadiyya, becoming a new sect. Born on the crossroads of the world’s Western and Eastern faiths, they also see Krishna and Buddha as prophets, and this varied religious palate perhaps explains their instinctive tolerance.

The Ahmadi have had a presence in Britain since 1913. In 1926 money raised by Indian women selling their jewellery paid for the beautiful Fazl mosque in Putney, the oldest purpose-built mosque in London, which was used as a shelter during the Blitz. Today there are 30,000 followers in Britain.

They have always been keen on promoting the positive side of Islam. Many Londoners will recognise the slogan “love for all, hatred for none”, which occasionally appears on buses, but the 21st century has brought a new need for public relations. As Dr Basharat Nazir, the UK press secretary, says: “Since 9/11 people have become frightened of Islam. We recognise and appreciate that. We’re saying there is an issue, but this is not what true Islam is.” He laughs: “When we started collecting for the poppies three years ago people were worried — they thought we were up to something else.”

The Ahmadiyyas believe that Islam has been distorted down the centuries, and in addition to their opposition to religious violence they place great emphasis on questioning and learning. Because of this they came to be highly literate and were disproportionately well-placed when partition came to India.

“We were very much involved in the creation of Pakistan,” says Rafiq Hayat, national president of the Ahmadiyya in Britain. “At that stage the community had a lot of influence.”

All this came to end under President Zulfikar Ali Bhutto, who was “sucked in by the mullahs” and had the Ahmadis declared non-Muslim — something, Hayat says, he had no authority to do. Many Muslims today do not consider them Muslims, although theologically they are perhaps closer to mainstream Islam than, for example, Baptists or Quakers are to Catholic Christianity. Perhaps a better comparison is with mainstream Judaism and Christianity in the first centuries after Christ; maybe the Ahmadiyyas will one day separate to form an entirely different faith.

The irony is that the Ahmadiyyas are exactly what optimistic liberals hope and imagine Muslims in general to be. When Muslims deny them as co-religionists they provide an argument for Westerners who say that Islam is incompatible with secular, pluralist democracies. Many Muslims appreciate this, but there are problems.

Born in Kenya, Hayat came to Britain in the late 1960s when Kenyan Asians were granted British passports after the government there turned on them. He is critical of much of Western foreign policy, especially our cosying up to dictators and Islamists, but he still supports Britain and British troops: “I believe we have to demonstrate loyalty, and poppies are a declaration of loyalty.”

The Ahmadiyya story in Britain demonstrates what went wrong and right with immigration after the war. Newspaper clippings of the opening of the Fazl mosque show a local crowd who came to celebrate and gawp at the curiosity; they’re smiling, curious, and entirely white. Although the BBC constantly tells us that Britain has always been a nation of immigrants, the demographic change of recent years has been remarkable. Under New Labour gross immigration peaked at more than 600,000 a year, and migrants from the Blair/Brown years account for 8 per cent of England’s population. Most of them are settled.

DNA research by geneticists such as Bryan Sykes and Stephen Oppenheimer suggests that the Anglo-Saxons account for about 5.5 per cent of English DNA, the Vikings a slightly smaller figure and the Normans probably around 2 per cent at most. So postwar immigration has made a greater impact on English DNA than all the migrations from the building of Stonehenge to the Great War.

Yet England has had waves of immigrants, most, like the Ahmadiyya, small groups of persecuted minorities. The most famous are the Huguenots, who settled in places like Wandsworth in south-west London, also the first home of the Ahmadiyya; Wandsworth’s coat of arms includes teardrops representing the sadness of the French Protestants at leaving their homeland. They also settled in Spitalfields, in east London, perhaps the only part of England that really does have a long history of migration and which to the diversity industry has become something of a modern-day Walsingham.

Spitalfields would also be settled by another persecuted minority — Russian Jews who fled the tsar from the 1880s onwards. Because both of these groups did well in Britain, and well for Britain, it has become easy to see it all as part of a narrative of one great nation of immigrants, but there are many differences. One is sheer numbers — Huguenots and Jews each accounted for about 1 per cent of the population, so that although there were issues with Jewish settlement in the early 20th century that superficially resemble the concerns of today’s natives, the differences are noticeable. Under Labour, partly because of immigration changes that were made to win favour with community leaders, the total Muslim population of Britain doubled to 4.6 per cent. It will pass 8 per cent before the end of the next decade.

Such numbers are bound to cause difficulties. White Britons are now in a minority in London, Luton, Leicester and Slough. Morden is one of those areas where the majority are leaving, although not on the scale of Barking and Dagenham, where the white British population fell by a third in the last decade, or Newham, where they now account for just 16 per cent of residents.

There have also been some terrible mistakes in the way that migration has been handled. Soon after the Jews arrived in east London a number of prostitutes were murdered by a still unknown male to whom an enterprising journalist (most probably) gave the name Jack the Ripper. Rumour spread around Spitalfields that the killer was a Jew, and the Jews prepared for the worst. Such accusations were nothing new in Russia, and they would inevitably be followed by the authorities unleashing a massacre. But in the morning the city went on as before, and though rumours persisted the newcomers realised that here was a country where the rule of law trumped prejudice and hatred, and minorities could prosper as equal citizens.

It was also a culture with a framework into which immigrants could integrate. Britain’s long-established Sephardic Jewish community, embarrassed by their newly-arrived co-religionists and nervous about the prejudice they might trigger, created the Jews Free School, a quite phenomenonally successful institution which had as its remit that it would take little Poles and Russians and turn them into English gentlemen. And so it did.

A century later, when immigrants from the Commonwealth were arriving in much larger numbers, British culture had changed. It was not that Pakistani immigrants did not want their children to become English gentlemen, but that Englishmen no longer knew of or believed in such an idea. From the 1970s the education system introduced what would become known as multiculturalism, an anti-intergrationist philosophy that encouraged the children of migrants to cling to their own identities. It was a disastrous policy.

Multiculturalism took off outside education, too. One of its many tragedies is that minorities coming to Britain to escape persecution have often met more of their tormentors here, funded by the English. Imagine that, as well as taking in Russian Jews from the 1880s, we had also taken in many more Russian Orthodox, and financially assisted their most conservative leaders, ignoring their inflammatory language with a pat on the head and a “Well, that’s their culture.”

Pakistan’s politics are now intertwined with Britain’s. After Bhutto was overthrown by General Muhammad Zia ul-Haq and then hanged, life became increasingly tough for minorities. “Zia ul-Haq was a fundamentalist,” Hayat says. “He passed a lot of laws that made it difficult. If you want to apply for a passport, you almost have to declare our leader as a heretic. We are worse off than the Christians.” The trouble, as many Pakistanis will tell you, is Saudi Arabia. “They’re the ones creating the biggest problem,” says Hayat. “There were only 200 madrassas when Pakistan came into being. Now there are tens of thousands.” And just as the religion of Saudi Arabia has spread to Pakistan, so the politics of Pakistan are brought to the streets of Britain.

Jamaat-e-Islami, a Pakistani party that opposes secularism, has many ties with the Muslim Council of Britain, which became influential under Tony Blair’s government (the Home Office and MCB even shared the same speechwriter). When Baitul Futuh was opened in 2003 Sir Iqbal Sacranie, head of the MCB, refused to recognise it as a mosque. Hayat says the MCB is “not representing Muslims” in Britain.

And while Baitul Futuh does not enjoy good, or indeed any, relations with neighbouring mosques, it had “a big issue” in particular with one in Tooting. “The imam there was very radical, he was making fatwas against our community,” says Hayat. “The government made it worse, this Tooting mosque got a lot of money.” And this was part of New Labour’s “anti-extremism” initiative. The Morden mosque, in contrast, was paid for entirely by donations.

Such is the intolerance that there were protests by Sunni Muslims against the building of an Ahmadiyya mosque in Walsall, in the West Midlands. “This was the time when minarets were banned in Switzerland, and at the same time there were photographs showing hundreds of bearded men in Walsall with placards saying ‘no mosque’,” says Hayat. There are also problems at universities, with radicalised students boycotting Ahmadi on campus and even making death threats. “You’d be surprised how radicalised some of our universities are,” Hayat remarks.

Most British Muslims, he says, are open-minded, but he adds: “Foreign-born imams are a huge problem, and they’re all Saudi-funded. The Saudis control all the strings in Pakistan, the Saudis are the major cause of conflict, including the syllabus being exported to Islamic schools here.”

Any problems the Ahmadiyya face in Britain are minor compared to those in Pakistan, where attacks are frequent. In May 2010, gunmen opened fire on one of their mosques in Lahore, killing 86 people. So they have to take security precautions in Britain. “We had 7/7 and every week we hear of police bringing people to courts,” Hayat laments.

But none of this deters them from their peace conferences and their charity collections for the British forces, which they will do again this year. They feel honour-bound. As Dr Nazir points out, they want to display their appreciation for Britain. “The poppy is about showing your loyalty. We like it here, everyone is free,” he says. “This is a lovely country.”