Another maverick defects. So what?

Maverick MPs who defect from one party to another tend to be just that: mavericks. Bob Spink, whose long-running battle with Essex Tories yesterday prompted him - contrary to assurances just a month ago - to become Ukip's first Westminster MP, is no exception.

Spink is against abortion and drugs, homosexuality, sex education and Europe. He favours capital punishment and once said of immigration policy: "What bit of 'send them back' don't you understand Mr Blair?" But he was also the only Tory MP last year to vote with Labour's Frank Field to retain the 10p rate of income tax for the kind of lower-paid workers he represents on Canvey Island and Benfleet. A self-made working class Tory with a PhD, he came up the hard way.

But fellow MPs are tough on defectors. Labour and Lib Dems shudder at a right-wing Essex populist who lost his seat in 1997, but won it back against the tide in 2001, boosting his majority to 8,201 (18%) in 2005. Tory colleagues say he used to be quietly sensible and ignored, but changed as he and party activist chums in Essex savagely fell out.

"They're all mad," says a senior MP. Spink was set to be deselected, lost the whip (he claims he resigned it) and was welcomed yesterday as "hard working and deeply principled" by Ukip leader Nigel Farage. Not even the excitable Tory bloggers could get very excited. They had seen it coming. "Never fall out with your local party," is an iron rule of politics.

But does it matter that Ukip now has a voice in the Commons? Probably not. Ukip's "quit Europe" position is strong enough in the Conservative ranks to have forced David Cameron and William Hague to back referendum calls on the Lisbon Treaty, a campaign which Tory donor Stuart Wheeler took to court yesterday. Unlike Ukip the Tories do not promise to reopen the issue if they win in 2009-10.

In any case, local Tories campaigning for the May 1 council elections hope to profit from the perception that Ukip is on the slide, sullied by scandal and divided. In 2004 it won 16% and 12 seats in the Euro-elections. In 2005 it averaged 2.8% in seats it fought, up on the 2.1% it won in 2001, but Jimmy Goldsmith averaged 3.1% in 1997. The stardust briefly provided by Robert Kilroy-Silk's membership ended in the usual micro-party personality split, a breakthrough opportunity lost.

The unkind truth is that whatever the motive - lofty or low - MPs who lose the party whip and defectors (six since 2005) rarely matter except in terms of parliamentary maths and public perceptions of disunity. Right-wing Empire Loyalists or left-wing Bevanites of the 50s usually rejoined the party or faded away. Even the 28 Labour-turned-SDP MPs who thought they were "breaking the mould" of two-party politics in 1981-82 lost their seats, mostly in 1983.

Only John Horan remains an MP: today a Tory one. At least he became a minister, a better reward than the peerage more often dangled before defectors. The odds are poor: Reg Prentice and Alan Howarth became ministers, then peers. Most don't, though Shaun Woodward, Tony Blair's most important pawn capture (1999), ended up in cabinet, a lucky fluke.

All that is above Bob Spink's pay grade. He broke the iron rule. "Bob says he's not changed, it's the party that's changed. We're happy with that message," beams a Cameron loyalist.