DAY five of the time to do and the new Blair government's promise to be radical in office becomes a shade more convincing. We knew from the start that, if implemented as pledged, constitutional change would form the real core of New Labour's radical agenda.

We certainly did not know - particularly in the final 10 days of the campaign when Labour was going around scaring little old ladies with the prospect of a fifth Tory government taking their state pension away from them - whether a Blair administration would be remotely radical on reforming the burgeoning welfare state.

It is a mistake, no doubt, to read too much into ministerial appointments before the MPs concerned have had any chance to impress by deeds. But sending Donald Dewar to the Scottish Office and Frank Field to social security - in what seemed designed to look like a joint venture appointment with the real secretary of state Harriet Harman - are the clearest possible signals, in these very early days, that Prime Minister Blair means to deliver a constitutional revolution in Britain and has a fundamental shake-up of welfare provision firmly in his sights.

The promised package of root and branch reform of the existing British state - home rule for Scotland and Wales, the removal of the voting power of hereditary peers, a freedom of information act, and a referendum on proportional representation - was always substantial by any standards. Much less convincing was Labour's degree of commitment, under fire, to some of its strands.

The devolution promise, in particular, has steered a defensive, zig-zag course these past two years, under Michael Forsyth's tartan tax fusillade. Now, with not a single Tory MP left north of the border, the Scottish Tories will be hard-pressed to mount any credible campaign in this autumn's referendum. Indeed a vocal faction in that shattered party is already calling for an early policy rethink.

An overwhelming majority of Scots voted for parties advocating change. Defence of the Union as is, despite John Major making it a centrepiece of his personal campaign, attracted less than 18% of Scottish voters, remember. So the only challenge left, it seems to me, is to get enough people to come out and vote yes: yes to make the popular mandate for home rule look credible.

No leading Labour politician is better equipped to do that than Dewar. Anyone who knows George Robertson personally knows that he, too, is a committed home-ruler. But Robertson inevitably carried significant collateral damage from his skirmishes with Forsyth. Even the Sunday before polling, he left those of us who were with him on a radio programme perplexed about precisely how long a Labour-controlled Edinburgh parliament would desist from using the proposed tax varying powers.

By the simple fact of being in the chief whip's office at the time, Dewar comes to the task of delivering the referendum and the home rule legislation with none of that baggage. His commitment, over decades, to the cause of a Scottish parliament is unimpeachable. Tony Blair, who personally seems to find the traverse of the Scottish question a more daunting challenge than a frontal assault on Downing Street, could not have chosen a better way of saying to the doubters: I really mean to deliver this.

It is much less clear how enthusiastic New Labour is about delivering another aspect of constitutional reform - a voting system at UK level which more accurately reflects the views of the electorate. As it surveys the sheer magnitude of its triumph, there is an obvious danger that New Labour will contemplate putting the promised PR referendum on the back burner. Tony Blair has already said he is not convinced by the arguments for change.

But, if anything, these arguments have grown since last Thursday. Despite the crushing nature of its victory New Labour should reflect on the manifest injustices in the outcome. Half a million Tory voters in Scotland are totally disenfranchised. The Liberal Democrats on little more than half the popular vote than went to the SNP captured 10 seats on less than 47% of the votes cast. At UK level, New Labour took nearly 64% of all the seats on less than 45% of the popular vote.

If for no other reason than that, one day, the electoral boot will be on the other foot, a Blair government that claims it wants to be radical should push ahead with the promised PR referendum. If it's good enough for a Scottish parliament, it's good enough for Westminster. Indeed I would go further.

Since there are now so many Labour MPs that they will have to occupy benches on both sides of the Commons chamber, the Blair government should redirect the money earmarked for the giant end-of-the-twentieth century party at Greenwich into building a brand new chamber to house the Commons in the next millennium. And it should be built in the round, breaking for ever the antique symbolism of political opponents kept two swords' length apart, reduced to trading verbal abuse across the void.

It is much too early to assess how specific policies will evolve under Labour. We know, because Tony Blair has said it often enough since Friday, that New Labour will govern as New Labour. That is, the immediate priority is to deliver the self-consciously restrained promises on school class sizes and health service waiting lists contained in the manifesto.

But Frank Field's role at social security points to much more radical thinking in the medium term. Since Peter Lilley first unveiled the Tory Pensions Plus proposals in March, I have been predicting that New Labour, in office, would eventually have to drop its opportunistic opposition to some of Lilley's core ideas if it was serious about fundamental reform of a creaking welfare system.

I wasn't alone in that view. Writing in the Telegraph at the time, Frank Field warned that Labour would have to move decisively ''to claim Peter Lilley's inheritance''. Labour's claims, during the campaign, that the Tories were out to privatise all pensions was cheap politicking. With Frank Field at social security, New Labour may soon be confronted with even more radical proposals, like compulsory individual savings schemes to cover ill-health and old age, than even Peter Lilley dared promote.