In his early days leading the Liberal Democrats Paddy Ashdown used to say that he would sell his grandmother for a bit of definition. It is hard for minority parties to impress their policies on the media and the public when they are frequently defined in terms of their relations with others.

Charles Kennedy takes over a party immeasurably stronger than that inherited by Mr Ashdown. They have 46 MPs, the biggest third party representation since the war, and a solid core of MEPs. They are strong in local government with around 5,000 councillors, and they are solvent.

But it will be hard for the new leader, succeeding somebody who had an 86% satisfaction rate in his own party, to establish precisely what the Liberal Democrats are for now that the New Labour government, with their active support, has implemented much of the agenda which they had been campaigning for, notably on the constitution.

Under Mr Ashdown, Mr Kennedy appeared to be one of the sceptics on the policy of "constructive opposition", working with the Labour government where their interests coincided, but during the leadership campaign he became the strongest voice in favour of its continuation, up to a point.

He will now have to establish just how far it is to go. He will have to show that the Liberal Democrats can still gain practical benefits from some association with Labour even without the personal chemistry enjoyed by Mr Ashdown and Tony Blair.

To make significant further progress the Liberal Democrats need proportional representation for Westminster elections. Only a Labour government could ever be expected to give them that, but Mr Blair has clearly cooled on the prospect.

An early test for Mr Kennedy will be whether he is prepared to withdraw his team from the joint Cabinet committee without some sign of a revived commitment from Labour to a referendum on PR.

Meanwhile he will have to make good his claim to ensure that at the next election the Lib Dems are also seen as a strong, independent, progressive alternative to the Labour Party.

As a new leader he will be helped by his strong media profile, by the fact that the leadership contest was remarkably good-tempered, and by the fact that he is a new act in town.

But aides say his is a collegiate style and it will take him time to develop the personal authority enjoyed by Mr Ashdown, whose military expertise and strong interest in the Balkans helped to make him a figure on a wider political stage.