What is the problem?
Since the Scottish Assembly started up in May 1999, the Scots have had control of their own health, education, agriculture and justice systems. Other matters, like foreign affairs, continue to run from Westminster. Scottish MPs continue to sit in the House of Commons and can vote on any legislation there, whether it applies to Scotland or not.
In November 2003, the Government narrowly won a Commons vote on whether to introduce Foundation Hospitals in England. If the vote had been restricted to English MPs, whose voters were the only ones likely to be affected by the legislation, it would have been defeated. In fact, the legislation was rescued by 44 Scottish Labour MPs who backed the Government.
Even more controversially, in January 2004, a Bill allowing English and Welsh universities to charge variable tuition fees, which did not apply to Scotland or Northern Ireland, scraped through the Commons by only five votes, the narrowest majority in the history of Tony Blair's government .
The only Scottish Tory MP, Peter Duncan, refused on principle to vote, because the issue did not affect Scotland. Almost all the other Scottish MPs - who numbered a total of 72 at that time - came out to vote, dividing 46-21 in the Government's favour, thus saving a very contentious piece of legislation. Why - people ask - should Scottish MPs be wielding a decisive vote on legislation that does not apply to the people they represent?
Where does West Lothian come in?
When the Labour government tried unsuccessfully to introduce devolution in Scotland in the 1970s, one of their most dogged opponents was the Scottish Labour MP Tam Dalyell, who continually brought up the anomaly that Scottish MPs would be able to vote on issues like health where they affected England, but English MPs would not be allowed to vote on health issues as they affected Scotland. Dalyell was, at that time, MP for West Lothian. Hence the "West Lothian question".
Why are we talking about this now?
In their election manifesto, last year, the Tories promised to make sure that "English laws are decided by English votes". It was thought that this policy might be quietly shelved after David Cameron established a Democracy Task Force, headed by the former Chancellor, Kenneth Clarke, to revisit issues like this. But at the weekend, Mr Clarke told a Sunday newspaper that there was no question of the Tories backing out of their commitment to the English. This was backed up by a hint from Mr Cameron's official spokesman that the Conservative leader is in favour of the idea.
The Conservative grievance is not just that individual pieces of "English" legislation get through on the strength of Scottish votes. Some Tories claim that they "won" last year's general election in England, because 8.1 million people in England voted Conservative, while 8 million voted Labour, although Labour won an outright majority of English seats. They say that there is a growing sense of resentment among the English at having their lives run by Scots.
One shadow minister, Alan Duncan, suggested at the weekend that devolution makes it politically impossible for a Scot to be Prime Minister. This is a good way of getting at the Tories' number one target, Gordon Brown, and at Menzies Campbell.
How would 'English votes' work?
After the trouble they had over devolution in the 1970s, Labour thought they had the answer worked out. They would cover England with regional assemblies, thus bringing devolution to England. They did, in fact, create the London Assembly. But the first attempt to create a regional assembly outside London, in the North East, was torpedoed when a local referendum produced a resounding "no" vote. The Government has not risked holding a referendum in any other region.
The most radical proposition is that there should be a separate English Parliament, which might meet outside London, say in Manchester. But the polling evidence says that this would be unpopular with the English, who would see it as an extra layer of bureaucracy and likely to break up the United Kingdom.
The compromise with the best chance of being adopted would be for the Speaker to designate certain days in the Commons calendar as "English" days. On those days, the Scots would have holiday while English MPs debated legislation that did not apply to Scotland.
What are the constitutional implications?
Even if the political party that controls the Commons also holds a majority of English seats, there could be arguments about what constitutes "English" legislation. For instance, suppose the government decided to abolish the London Assembly - is that an "English" or a "UK" matter? By what logic does it concern the MP for Berwick, in the far north of England, but not his parliamentary neighbour in Berwickshire, on the other side of the border? But the real difficulties would begin if there was one party or coalition in control of the UK Parliament, while a different party or coalition controlled a majority of English seats. You would then have a Prime Minister who could not introduce legislation affecting the public services for 85 per cent of the population, and an opposition leader who controls these public services but has no formal position in the government. Would the English MPs be allowed to make decisions that cost money, and how would they raise the necessary taxes?
What about Wales and Northern Ireland?
At present, none of this applies to Northern Ireland, since its Assembly was suspended on 14 October 2002. None of the British parties disputes the right of English, Scots and Welsh politicians to run the affairs of Northern Ireland. But the West Lothian Question is relevant to Wales, which has its own Assembly, though its powers are not as great as that of the Scottish Parliament. In 2003, 24 Welsh Labour MPs voted with the Government when it was proposing to introduce Foundation Hospitals in England, thus helping to rescue the legislation after the Welsh Assembly had decided that there would be no Foundation Hospitals in Wales.
Should there be 'English votes for English laws'?
* Scottish MPs vote on health and education bills that affect England but not Scotland.
* A Scottish Prime Minister would bring in laws that do not apply to the people who sent him to Parliament.
* Devolution was a bad idea, but now it is here it should apply equally to the English.
* Public services that affect 85 per cent of the population could be controlled by the leader of the opposition.
* The UK is not made of four equal parts, but one big country with three small partners.
* Home rule for England would mean the disintegration of the United Kingdom.