A year ago today, Prime Minister David Cameron greeted the result of the independence referendum with the words: "The people of Scotland have spoken."

He said it would "have broken my heart" to see the UK broken up but he also paid tribute to Yes Scotland and sent a message to the 1.6 million Scots who voted for independence. "We hear you," he said.

He went on to talk about his commitment to devolving more powers to the Scottish Parliament and, in his early-morning statement delivered on the steps of Downing Street, announced Lord Smith of Kelvin would oversee a cross-party process leading to new legislation.

The Scotland Bill that grew out of the Smith Agreement is now before the Commons.

It will transfer extensive tax powers, including almost full control over income tax and a £2.7billion welfare budget to Holyrood.

The Bill will move to the Lords towards the end of the year before returning for a final vote in the Commons around February or March.

At the same time, MSPs will vote on whether to accept the new powers, which would become available around 2018.

It seems inconceivable a Nationalist government would turn its back on a deal giving it responsibility for raising more than half its budget but that's exactly what Nicola Sturgeon and John Swinney are threatening to do.

There may be devils in the detail, they warn, and in recent weeks have both stressed they will not accept the new powers unless the overall package is "fair to Scotland".

The detail belongs to a thing called the fiscal framework, which is being thrashed out between the Scottish and UK governments.

Essentially it will reset Scotland's budget to take account of the new tax powers. The trick (and no-one denies it's tricky) is to devise a system under which the Scottish Government reaps the rewards or suffers the consequences of its own decisions, while Scotland's share of UK resources continues to be determined by decisions at Westminster and the strength of the UK-wide economy.

There are different views on how this can be achieved.

To begin with, the Scottish Government's £30bn block grant from Westminster will be cut by about £18bn, equivalent to the revenues from all the newly devolved taxes. But how should the grant be calculated in future years? There are various possible formulae, some more advantageous to Scotland than others.

Then there is the question of borrowing powers. The Scottish Government needs the power to borrow, not least to help it iron out fluctuations in in tax revenues. But how much? The Scottish Government wants to maximise its ability to borrow; the Treasury is conscious of the impact reckless Scottish borrowing could have on the UK's balance sheet.

In its final report before the general election, Westminster's Scottish Affairs Committee warned of a "huge potential for grievance" over the fiscal framework. Ms Sturgeon's threat to block the Bill has now set alarm bells ringing even louder.

In a recent interview with The Herald, former Chancellor Alistair Darling said the negotiations over the fiscal framework should be conducted in public to prevent it from becoming a political football.

Sir Tom Hunter, the entrepreneur, said the fiscal framework "will undoubtedly fuel consistent sniping that Scotland is not getting enough of its share".

UK Government sources insist the talks are progressing in a businesslike fashion. Officials have held a series of talks in Edinburgh and London and the two governments' lead ministers, Mr Swinney and Chief Secretary to the Treasury Greg Hands, have already met face to face as part of the process. The intention is to seal a deal within the next six to eight weeks.

David Mundell, the Secretary of State for Scotland, believes Ms Sturgeon is bluffing, a hunch no doubt based on the SNP's response to the previous Scotland Bill, which was opposed vigorously until it came to a vote in the Holyrood chamber.

But four years on from that, the SNP have a majority at Holyrood and Ms Sturgeon would relish a battle with Mr Cameron in the run-up to next May's Holyrood election.

If hard-bargaining turns into a full scale battle, delaying the introduction of the Scotland Bill until after the poll would also remove the need to answer difficult questions about income tax rises or changes to benefits.

It is unclear quite how things will play out but don't bank on the Scotland Bill passing without a hitch.