Image caption On 1 October, 2009, the UK Supreme Court came into being

The quashing of Nat Fraser's conviction is a dramatic twist in a long-running murder investigation.

But it is also a significant moment in Scotland's judicial history.

This is the first time the Supreme Court in London has, in effect, directed a Scottish court to quash a criminal conviction.

It has prompted a bad-tempered spat among lawyers and politicians about where supremacy rests in Scots Law.

Article XIX of the Act of Union 1707 is clear on the matter, stating that courts in Westminster "shall have no power to Cognosce Review or Alter the Acts or Sentences of the Judicatures within Scotland or stop the Execution of the same."

Despite being set in ink in the treaty which united England and Scotland, this was quickly ignored when, in 1711, the House of Lords established itself as the final court of appeal in civil cases, overruling Scotland's Court of Session in the case of Greenshields v Magistrates of Edinburgh.

But for three centuries the High Court of Justiciary, sitting in Edinburgh as a Court of Criminal Appeal, remained the ultimate arbiter of all Scottish criminal cases.

That continued until the advent of devolution and human rights legislation. On 1 October 2009, after a hiatus involving the Judicial Committee of the Privy Council, the UK Supreme Court came into being.

It sits in the former Middlesex Guildhall, on the western side of Parliament Square in Westminster, and consists of 12 justices - including two Scottish judges, Lord Hope of Craighead and Lord Rodger of Earlsferry.

Nat Fraser case Arlene Fraser, 33, disappears on 28 April, 1998

Nat Fraser is convicted of her murder in January 2003

The killer lodges an appeal against his conviction in December 2003

The appeal is granted in May 2005

Fraser is freed from prison pending an appeal in May 2006

In May 2008, Fraser loses his appeal Timeline in full

As well as taking on appeals in civil matters previously heard by the Appellate Committee of the House of Lords, the Supreme Court considers cases relating to Scotland in two distinct, but limited, areas.

The first involves issues of "constitutional importance" - whether the Scottish government is acting within its powers, whether it is failing to comply with a duty imposed on it, or whether the Scottish Parliament is legislating within its competence as set out by the Scotland Act 1998.

If, for example, Holyrood passed a law ordering the scrapping of nuclear weapons on the Clyde this could be challenged in the Supreme Court on the grounds that defence is a matter reserved to Westminster.

The second area for the Supreme Court relates to human rights law in criminal cases.

It was this which formed the basis for Nat Fraser's appeal and for an attempt by Luke Mitchell to have his conviction for murdering his 14-year-old girlfriend Jodi Jones in Midlothian in 2003 quashed.

Mr Fraser had argued that the failure of prosecutors to reveal doubts about a crucial piece of evidence had infringed his right to a fair trial under Article 6 of the European Convention on Human Rights.

The Supreme Court agreed, ruling that the "trial would have been significantly different if the undisclosed evidence had been available".

Quash conviction

It concluded by saying a new trial would have to be considered, although it added: "This matter is best dealt with by the High Court of Justiciary. So the case will be remitted to that court for this question to be dealt with, and it will be for that court to quash the conviction."

Ben Wilson, deputy head of communications for the Supreme Court, was keen to point out that the justices were asked to determine whether Mr Fraser had received a fair trial under European law not whether the conviction should be quashed.

The court was not, he insisted, "supreme" in Scottish criminal matters, despite its name.

Nonetheless Mr Wilson conceded that the judgment left the High Court "effectively bound" to quash the conviction.

The only other case the Supreme Court has sent back to the High Court is that of Cadder v HM Advocate, which was remitted for "further procedure" rather than with a "direction to quash".

Image caption The Nat Fraser appeal was delivered from the Supreme Court at Middlesex Guildhall, opposite Westminster The Nat Fraser appeal was delivered from the Supreme Court at Middlesex Guildhall In pictures: UK Supreme Court

The decision in Cadder that Scottish police could no longer question suspects without a lawyer present went against a ruling by seven Scottish appeal court judges and led to a splitting headache for police, prosecutors and politicians, a change in the law and a flurry of similar appeals, including Luke Mitchell's.

Since then, the difference in the Supreme Court's and the Appeal Court's interpretation of European law has generated a storm of controversy.

First Minister Alex Salmond has led the charge, portraying Scots Law as under threat from an alien jurisdiction with the power to "throw open cell doors".

He has chosen to focus his attack not on Strasbourg, the home of European human rights law, but on London where the Supreme Court is, he claims, "second guessing" Scotland's highest criminal court of appeal.

Former Lord Advocate Lord Fraser of Carmyllie QC is among his supporters, saying Mr Salmond is "spot-on" in his criticism and warning of a "drift towards the loss of that distinctive identity of Scots Law".

I think the power of the Supreme Court is increasing, it has increased, and it's now time for it to be diminished Alex Salmond, First Minister

But other lawyers and political opponents have accused the first minister of playing to the gallery for political purposes, of using an unpopular judgment, based on human rights legislation which he voted for, to manufacture a row between Edinburgh and London.

In particular Mr Salmond has been charged with undermining the independence of the judiciary with specific attacks on Lord Hope and Tony Kelly, a lawyer specialising in human rights.

And, the critics argue, if the first minister had his way human rights appeals would be heard not by two Scottish judges at the Supreme Court in London but by no Scottish judges at the European Court of Human Rights in Strasbourg, making justice tardier, costlier and more remote.

Mr Salmond's answer has been to appoint a panel of experts to review the situation. Its chairman, Lord McCluskey, is an intriguing choice.

On the one hand the former solicitor general has publicly criticised the Scottish Justice Secretary, saying Kenny MacAskill - who is also a lawyer - should be ashamed of himself for accusing the 12 Supreme Court justices of "ambulance-chasing".

On the other hand Lord McCluskey has been a leading critic of the European Convention on Human Rights.

In an article for Scotland on Sunday in February 2000 the judge wrote that the impact of the Convention on Scots Law was already "devastating" and suggested that the legislation was providing "a field day for crackpots, a pain in the neck for judges and legislators, and a goldmine for lawyers".

But there are those, like human rights lawyer John Scott, who believe the law should be a pain in the neck.

"We should welcome suggestions of ideas for how to make our system fairer and not simply sit back and say our system's perfect as it is," he said.

"Effectively what the Supreme Court is doing is anticipating how these matters would be looked at by the European Court of Human Rights.

"This is to save someone having to make the trek to Strasbourg."

Mr Scott insisted Scottish lawyers were "not frightened" of the Supreme Court and accused Mr Salmond of an approach which represented "narrow-minded nationalism".

The first minister brushes aside such criticism with typical bluntness.

"I think the power of the Supreme Court is increasing, it has increased, and it's now time for it to be diminished," he said.

With opinion so divided, even by a judge's standards rather a lot is hanging in the balance of Lord McCluskey's review.