It is not complicated, although some claim it is. A constitutional convention is not needed. Whatever settlement is thought right and appropriate for the Scots, then exactly the same settlement should be right and appropriate for the English, the Welsh and the Northern Irish. Sauce for the goose is very much sauce for the gander.
The devolution of more power to the Scottish Parliament must mean devolution of more power to the Welsh parliament. The English Parliament can comprise English MPs voting on purely English measures in the House of Commons. The Scots, Welsh and Irish MPs could and should abstain on such votes. In matters where the English and Welsh are passing laws that exclusively affect England and Wales, then they alone should vote on them.
The whole Parliament should deal with any matters that may affect the whole of the United Kingdom: this will end up comprising defence, income and corporation taxes, capital gains and inheritance taxes (the UK should have a single tax regime) defence, wars and foreign adventures, and immigration.
There will inevitably be anomalies and inconsistencies, but these happen in every constitution; the trick is to have the least worse arrangement, because perfection is impossible.
Of course the Labour Party have a vested interest in having Scottish MPs vote on all matters in the House of Commons, and it seems to me that the Labour Party is desperately seeking some justification for this present arrangement; there is none, in democratic terms,. The only justification is to put or keep the Labour Party in power.
I see no reason why changes to produce a fairer more democratic constitution should not be in place before the next General Election in May 2015.