Suddenly, the UK Independence Party is the wild card of British politics. It left the Conservative Party bruised and traumatised by beating it in the Eastleigh by-election. It is tipped to win a large share of the vote in next year's European elections, may well cost David Cameron's party victory at the next general election, and is the force whose electoral magnetism many say will pull the Tories ever rightward. It is also the party – its programme and personalities – that has so far escaped scrutiny. Until now.

If Ukip had a name that truly reflected its priorities, it might be called the UK Immigrationphobe Party. Ostensibly the anti-EU party, an obsession with immigration and exit from Europe as a means to close Britain's doors is its prevailing motive. The word immigration runs through its policy statements like red lettering in seaside rock, and its proposed five-year ban on entries to the UK is the message it rams home on every doorstep.

That is far from all. It is deeply sceptical of global warming, wants to abolish inheritance tax, employers' National Insurance contributions, aims to partially reverse the recent hunting and smoking bans, and would increase defence spending by some 40 per cent. It is, in thought if not yet in personnel, the extreme right-wing of the Conservative Party in exile; a party run in the main by self-made businessmen with an agenda to match. And it has a record of defections, internecine squabbles and acrimony, plus scandals that have led two of its former MEPs to jail.

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Founded in 1991 as the Anti-Federalist League by Dr Alan Sked of the London School of Economics (it became Ukip two years later), it has had, in the past 20 years, no fewer than eight leaders. And the upper reaches of its 22,000 members are fed – and depleted – at regular intervals by defectors coming in and going out, among them its founder. Two of the people elected as Ukip MEPs have since defected to the Conservative Party, and Nikki Sinclaire, elected a Ukip MEP in 2009, was expelled from Ukip for refusing to be part of the right-wing Europe of Freedom and Democracy grouping in the European Parliament.

On the other side of the account, two of its three peers sitting in the Lords were defections from the Conservative Party; its one Member of the Legislative Assembly in Northern Ireland is David McNarry, who was expelled from the Ulster Unionists; and the only person ever to represent the party in the Commons was Robert Spink, who defected from the Conservatives in 2008.

All these comings and goings mean its headquarters must at times resemble a tourist hotel at the height of the season. The party is, in fact, run out of an office on a business park in Newton Abbot, Devon. Its latest available accounts, for 2011, reveal it has 12 employees, made up of four party officers and eight administrative staff – four fewer than in the previous year. The Ukip leadership has managed to turn around an £18,000 deficit and posted a profit of almost £100,000.

Its MEPs have been criticised for a less-than-stellar attendance record at the European Parliament. The latest data from Votewatch.eu says that Nigel Farage's attendance in plenary has been 141 days out of 196, placing him 708 out of 754 MEPs, and his deputy Paul Nuttall managed 123 days out of 195, ranking him 742nd. More embarrassing for the party was the jailing of two of its former MEPs. In 2007, Ashley Mote, who represented South-east England, was jailed on 21 counts of benefit fraud, totalling £65,000. In 2009, Tom Wise, former MEP for East of England, was jailed for two years after falsely claiming thousands in expenses.

Mr Farage has carved out a niche for himself as a political maverick who, with a nice line in good humour, plays well on Question Time. Insiders, however, say he is somewhat shy when it comes to detail. One former Ukip member said the party had attempted to put together a co-ordinated policy platform before the last election with 18 separate policy groups charged with contributing to the Ukip manifesto. But he added: "The whole thing was eventually binned. The Ukip leadership doesn't seem to be interested in details; it's a very successful single-issue pressure group. Nigel was very uncomfortable with a manifesto which stated that 'A Ukip government would do this or that'."

Marta Andreasen, the former Ukip MEP who joined the Conservatives during the Eastleigh by-election campaign, said the party was, in effect, a pressure group. "Ukip says it has policies on things other than Europe, but none of them have been properly costed. It is very easy to use immigration as a threat to the population of this country, because people are losing their jobs and the welfare state is being reduced. I agree that there should be proper controls on immigration, but Ukip's position on this – warning that millions of Bulgarians and Romanians would come to this country – was one of the things that contributed to my leaving the party. We were coming too close to the BNP. We were on the margins of the racists."

To the accusations that his party is covertly racist, Mr Farage highlights the fact that the Ukip candidate in the Croydon North by-election was the Jamaican-born former boxer Winston McKenzie. However, Mr McKenzie provoked protests when he claimed that allowing gay couples to adopt was "unhealthy".

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Ukip's deputy leader, Paul Nuttall, uses his blog to vent his spleen against political correctness. In one post, he supported the Bristol hoteliers who faced court action after refusing to let a gay couple sleep in the same bed, and the football pundits Andy Gray and Richard Keys, sacked by Sky over their "sexist" remarks about a female referee's assistant. He wrote: "Firstly, Christian hoteliers Peter and Rosemary Bull lost a court case in Bristol, which was brought by two homosexual men who took exception because the couple refused to allow them to sleep in the same bed. What a complete joke. Aren't people allowed to live by their religious beliefs in the country any more? … Gray was partaking in something we call 'banter' in football circles … It's not as if he said it live on air, but, then again, so what if he had?"

A Ukip spokesman denied the party was obsessed with migration and insisted that it had a policy platform which "reflected all the real concerns of the British people". He added: "We have policies on the economy, public services, Europe … everything. During the Eastleigh campaign, we expected to talk a lot about Europe, but when we knocked on doors people wanted to ask about immigration. Other people have analysed the election since and said immigration was the main concern of 55 per cent of the people who voted, so we were very much in touch with the people of that area. Ukip, of course, repudiates all types of racism and sectarianism. BNP former members are barred from joining Ukip."

Additional reporting by Jemma Crew and Katie Grant

Ukip policies

Lower taxes

"Abolish the tax on work – employers' National Insurance [NI]." This would lose around £50bn a year in revenue, at a stroke, reversing all progress in cutting the deficit since 2010.

"Take 4.5 million of low incomes out of tax with a simple, flat rate income tax … Merge 20 per cent basic income tax with 11 per cent NI to create a 31 per cent flat tax on all earned incomes over £11,500." This would create another vast hole in public finances, and mean tax cuts for all higher-rate taxpayers.

"Ukip have a long-standing policy to abolish inheritance tax." This would cost about £3bn a year.

"Global warming is not proven – wind power is futile. Scrap all green taxes, wind-turbine subsidies, adopt nuclear power to free us from dependence on fossil fuels and foreign oil and gas."

"Cut taxes on small businesses." Unspecified.

Lower public spending

"Public spending is increasing and the coalition's cuts do not scratch the surface of Labour's deficit. We must cut down government if we are to return to a sound economy."

Quangos: "Bring them under Parliament's control and cut the cost substantially". No definition of "substantial".

"Make real and rigorous cuts in foreign aid." The aid budget is about £10bn a year.

"By leaving the EU we save over £45m a day plus £60bn a year due to EU trade barriers, business regulation, waste, fraud." £60bn a year is equivalent to the annual spending of the entire Education Department.

Higher public spending

"Spend an extra 40 per cent on defence annually." A cost of £19bn pa.

"Double prison places to enforce zero tolerance on crime." Cost about £4bn pa.

"Roll all state pensions and benefits into a simple, substantial citizen's pension." Uncosted.

Restore student grants, but abandon target of 50 per cent of young people going to university. Uncosted.

A better yesterday

"Only by leaving the EU can we regain control of our borders."

"Life must mean life."

"Our traditional values have been undermined. Children are taught to be ashamed of our past. Multiculturalism has split our society. Political correctness is stifling free speech."

"Support grammar schools."

"End the ban on smoking in allocated rooms in public houses, clubs and hotels."

"Hold county-wide referenda on the hunting ban."

John Rentoul

Sources: 'What We Stand For, A Ukip Statement of Principles', 28 November 2011, and other documents on the Ukip website