Replacing council tax with a local income tax is the big idea in the LibDem manifesto - as it will be when the SNP unveils its manifesto today. It is one of those policies which echoes to the essence of fairness: the notion that we all contribute to the shared services on which all communities depend, in accordance with our ability to pay, is so self-evidently sensible that we wonder why no-one thought of it before.

However, the simplicity is deceptive. As always, the mechanics are the problem. While his criticism of council tax as "unfair and discredited" will strike a chord with many voters, the alternative depends on getting the sums right. Launching his party's manifesto for the Holyrood election yesterday, Nicol Stephen argued that 70% of people in Scotland would gain: particularly pensioners and those on low incomes. The LibDems have set their local income tax at between 3.5p and 3.75p in the pound, compared with the centrally-imposed 3p the SNP favours. However, the discretion offered by Mr Stephen and his colleagues is only 0.25p in the pound. Hardly maximum devolution of responsibility.

As with the SNP, the LibDem tax will be levied only on earned income, ignoring investment income. There is a bigger problem. Independent estimates have put the rate required to fund current local government services in full between 5p and 6p in the pound, allowing Labour and the Conservatives to attack "the black hole" at the heart of the LibDems' and SNP's local income tax policy. The Liberal Democrats have been in coalition with Labour in Scotland for eight years. Four years ago they were able to capitalise on claiming credit for two policies that were implemented in Scotland but not in the rest of the UK: free personal care for the elderly and the rejection of student tuition fees in favour of a graduate endowment. This time, their distinctive contribution to separate Scottish policy is having secured proportional representation in local government elections; a generally less populist cause, the effect of which will become apparent after May 3.

This time, too, they must answer for their part in a Scottish Executive with whose record the public is clearly underwhelmed. They are, therefore, pushing all the populist buttons: 1000 more teachers, 200 extra dentists and 2000 more nurses, plus tougher community sentences and faster trains. All this was delivered with a timetable for the first seven months in office, but longer-term plans include a commitment to 100% of energy from renewable sources, a commission to consider the division of power between central and local government, and a second constitutional convention to build consensus on new legislative and fiscal powers for the Scottish Parliament.

Refusal to hold a referendum on independence was the one issue which Mr Stephen declared "non-negotiable" yesterday, but the reiteration that they want more powers devolved to the Scottish Parliament is significant. This manifesto is so liberally studded with policies likely to appeal to each of the main parties, that it is difficult not to read this sunny vision of Scotland under the LibDems as a lonely-hearts ad. Mr Stephen's earnest statement that "we have delivered some of the key policies in the Scottish Parliament, some of the defining issues of the first few years of the Scottish Parliament" ended with his heart on his sleeve: "And now we want to go further." The unanswered question is: with whom?