Take a party in the throes of a hotly contested leadership election after messily ousting its leader. Take another, reinvigorated by the arrival of a sparkling new chieftain. And take a third, the governing party as it happens, whose impatient leader-in-waiting not only lives in the constituency, but dropped in from time to time to campaign. Who wins this supposedly predictable by-election, but the candidate for the party in turmoil, the Liberal Democrats.

Dunfermline and West Fife, where Willie Rennie overturned Labour's 11,000 majority to win by almost 2,000 votes, must now go down in the annals of maverick by-elections. The question is whether it will also join the likes of Fulham, Eastbourne and Ribble Valley in foreshadowing future trends. How much can be read from the runes of Dunfermline?

Local issues, of course, help to explain Mr Rennie's victory, as does the weakness of his Labour opponent. There was also a protest element: this election was not going to decide the fate of a prime minister, after all. Whether this heralds the start of a wider defection from Labour will only become clear with time.

Yet it undoubtedly conveys several smaller messages. It warns the Conservatives, whose candidate failed to increase the party's share of the vote, that David Cameron may not be the magic vote-catcher he seems, especially north of the border. He has more work to do.

A harsher message is for Gordon Brown. The Chancellor had campaigned in Dunfermline as a future prime minister. This vote can be seen not only as a rebuke to Mr Blair and his government, but as a warning shot to Mr Brown. The Chancellor can dismiss the result as a one-off, but he will ponder on this: if he cannot win in his own backyard, how much harder will he find it in the Home Counties of England?

The consolation, as enormous as it is unexpected, is for the Liberal Democrats. Charles Kennedy, for all his faults, remains an asset. And having no leader seems to be almost no liability for a party whose amateurishness is part of its charm. This should offer cheer to the party's next leader: not only is there another Liberal Democrat MP, but the recent disarray has been less damaging in electoral terms than even the bravest optimist dared to believe.