On 8 September Gordon Brown announced a plan to fast-track new powers for Scotland if voters chose to remain in the UK.

We’ll never know if the former Labour prime minister’s intervention swayed undecided voters, but he is set to be at the centre of a new devolution deal for Scotland.

The Conservatives and Lib Dems have also pledged to extend the Scottish parliament’s powers by January next year. As ever, the devil will be in the detail.

What has been promised?

Ed Miliband, David Cameron and Nick Clegg used that great shop window for democracy, the front page of the Daily Record , to set out a joint promise to the people of Scotland.

The three main parties agree that “the Scottish parliament is permanent and extensive new powers for the parliament will be delivered by the process and to the timetable agreed and announced by our three parties, starting on 19 September”.

The other concrete promise is to retain the Barnett formula system used to decide funding levels in the four nations of the UK?

What could possibly go wrong?

The timetable

There doesn’t seem to be any evidence of back-sliding here yet. Mr Brown says he has already secured a parliamentary debate on 16 October.

A month-long consultation period will come next, and all parties say new laws should be on the statute books by January next year.

Some backbenchers have already said they will vote against “devo max”, but cross-party agreement makes it unlikely that a new bill will come a cropper in the Commons.

Permanent parliament?

Mr Brown has said in the past that he wants to “write in the British constitution that the Scottish Parliament is permanent, irreversible and indissolvable”.

Note that the three party leaders have not actually agreed to this rewriting of the constitution – they have only said they “are agreed that the Scottish parliament is permanent”.

We don’t think that anything in law can ever literally be irreversible. It may be politically difficult for a future government to strip Holyrood of its powers, but is it possible to make a change “irreversible”?

The constitutional expert Professor Vernon Bogdanor told us: “It has never yet been done.

“The 1801 Act of Union with Ireland proposed that the Anglican Church be the established church in Ireland in perpetuity. But when Gladstone disestablished it in the 1860s, and someone brought a case against him, it failed.”

“There can be an obligation of honour, but no more.”

Extensive new powers?

We don’t know exactly what these will be, although we can make a fair guess.

Mr Brown made 12 proposals in his speech, many of them already trailed in Labour’s devolution commission report, published in March.

He offered Holyrood more control over: the Work Programme, housing benefit, the attendance allowance disability benefit; Scotland’s seabed and foreshore; capital borrowing, railways; income tax; employment rights; equalities legislation; health and safety; elections.

Some of these points were a bit light on detail, and Mr Brown doesn’t even appear to have published them in more fleshed-out form in a document so we can unpick the detail of what is on offer.

The assumption most commentators are making is that the proposals will be as set out in Labour’s devolution commission paper.

If that is right then it seems unlikely to be true, as the Daily reports, that Scottish governments will be able to borrow £22bn by issuing bonds.

Labour’s devolution commission recommends a more modest £2.2bn and it doesn’t like the idea of a Scottish government issuing its own bonds.

The level of control over taxation remains undecided. Mr Brown has refused to say what percentage of income tax will be spent in Scotland.

As far as we are aware none of Mr Brown’s 12 points are set in stone yet, but few are likely to be controversial with the other parties.

All three main parties have already committed to increasing Holyrood’s powers to raise more taxes, so a fairly swift compromise seems likely.

What is not on the table?

There are no concessions on defence, meaning the Trident nuclear deterrent is likely to stay on the Clyde for the foreseeable future.

Powers to raise and spend taxes will be extended but will be more limited than the full fiscal control promised by independence.

Scottish governments will be able to scrap the “bedroom tax” – as the SNP wanted to – but they won’t be able to make changes to the state pension.

Immigration will remain in the hands of Westminster so there won’t be any relaxation of the rules to encourage higher migration to Scotland – another SNP aspiration.

Barnett formula

The promise to carry on with the current system that decides public spending budgets in the devolved administrations is sure to remain a sore point.

Under current arrangements people in Scotland, Wales and Northern Ireland all get more public money per head than people in England.

This is not, as Gordon Brown told Channel 4 News this week , based “on the needs of the different parts of the country”.

It is not based on any assessment of need, it is simply an assumption that rises in public spending in England automatically lead to more money for the devolved governments.

The man who created the formula, Lord Barnett, has said his system is unfair to English taxpayers and should be scrapped.