Nicola Sturgeon used the first anniversary of Scotland’s independence referendum to call on David Cameron to keep the promise he made to voters a year ago.

“What people see at Westminster is a Tory government failing to fully deliver on the vow it made on more powers for our parliament”, the SNP leader claimed on Friday.

A new Scotland Bill, which gives the Scottish Parliament more powers to raise taxes and control welfare spending, among other things, is currently going through Westminster.

The Prime Minister insists that this means the so-called “Vow” he signed along with the then Labour and Lib Dem leaders on the eve of the #indyref is being implemented in full.

The SNP say they are so unhappy with the legislation as it stands and are threatening to block it.

Has Mr Cameron really reneged on the promise he made last September?

What was “the Vow”?

This was the famous promise made to Scots at the eleventh hour by David Cameron, Ed someone and Nick someone, as featured on the front page of the Daily Record:

As solemn vows go, this one was pretty vague: “permanent and extensive new powers” were on offer, but no more detail was forthcoming.

It was left to the Smith Commission to try to come up with the fine print – a long list of new powers to be handed over to Holyrood, as agreed by representatives of the major parties in November last year.

The British government says it is honouring the Smith agreement in full with its new Scotland Bill.

The SNP’s position on Smith was complicated: the party signed off on the agreement, but quickly said the proposals did not go far enough.

Now the argument centres on whether the Scotland Bill really translates Smith into law, or whether the British government is rowing back on the detailed promises it made last year.

How much power is on offer?

The Bill gives the Scottish Parliament the power to set rates and bands of income tax. Holyrood will keep all of the income tax raised in Scotland, and half the VAT. The British government says these and other new tax-raising powers are worth about £15bn.

New welfare powers are said to be worth around £2.5bn, and Scottish governments will have more control over roads, onshore oil and gas extraction, management of the Crown Estate and more.

Analysts from the Scottish Parliament Information Centre concluded that the fiscal changes set out in the Smith Commission would mean Holyrood was one of the most economically powerful devolved parliaments in the world, compared to federal systems like Germany and Switzerland:

Clearly, these are significant new powers, so it’s hard to say that the the letter of the “vow” has been broken – but that’s hardly surprising, given how vague that promise was.

Why is the SNP unhappy?

We asked the party for a list of its outstanding concerns about the Scotland Bill. Five of them concern welfare:

• The UK Government would have an effective “veto” over changes to Universal Credit and other parts of the welfare system
• The Smith recommendation for a power to create new benefits in devolved areas is “incredibly limited”
• The ability to top up reserved benefits has been watered down
• The Scottish Parliament would be prevented from creating additional benefits
• There are  “unwarranted restrictions” on carers’ benefits

Two of the objections relate to employment:

• The Bill limits the Scottish Parliament’s competency on services for disabled people and those at risk of long-term unemployment
• It won’t hand over control of all employment support services delivered by the Department of Work and Pensions, as promised

The final objection concerns the Sewel Convention, which states: “Westminster would not normally legislate with regard to devolved matters in Scotland without the consent of the Scottish parliament”.

The Smith agreement says this principle should “be put on a statutory footing”. But the SNP says:

• The Sewel Convention has not been put on statutory footing in the legislation, as Smith proposed – rather the existence of the convention is simply recognised.

It’s hard to say whether these things will prove to be real deal-breakers, or whether they can be hammered out before the Bill becomes law.

On some of the points, particularly the last one, even leading experts disagree on the rights and wrongs.

Consitutional lawyers are divided on whether the Scotland Bill actually makes the Scottish parliament permanent, or just says that it is.

David Cameron indicated today that the government will table an amendment to the Bill which may settle this point.

What happens now?

The Bill will return to the House of Commons soon for the report stage – which means MPs from all sides can table amendments calling for the bill to be changed.

The government has already voted down SNP and Labour amendments calling for full fiscal autonomy and new welfare powers, saying these fell outside the terms agreed by the Smith Commission.

But it is not impossible that further amendments could see a compromise reached between the British government and the SNP.

However, there is also a separate argument ranging about the “fiscal framework” surrounding the deal.

Because Scotland would get new powers to raise taxes, it would see the Block Grant it gets from Westminster reduced, and this is the subject of wrangling between the Treasury and the Scottish Government.

Scottish finance secretary John Swinney has threatened to block the Scotland Bill’s passage into law if a deal cannot be struck.

It is possible that the SNP could scupper the Bill by voting against it in Holyrood, which would mean that the process of further devolution that began with the signing of the “vow” just over a year ago could yet come to a shuddering halt.