"Today there is no Scottish Constitution," declared Andrew Dewar Gibb in his 1944 book A Preface to Scots Law.

And to write at length on the subject, added the veteran Scottish Nationalist, "would be something of an absurdity". It was nothing more than an historical curiosity, not a living, breathing thing.

But devolution has changed all that. As Professor Alan Page of Dundee University writes in his new book, Constitutional Law of Scotland, not only did the Scotland Act of 1998 set out a "Scottish constitution", it's been under constant revision ever since.

Of course Scotland (and indeed the UK) doesn't have a constitution in a codified sense, although it's far from "unwritten", as is often claimed. On the contrary, it exists in a myriad of documents, all of which receive attention in Professor Page's useful book.

Doubtless there will be a second edition, for several aspects of Scotland's constitution remain in flux, chiefly English Votes for English Laws (Evel), a Pandora's Box the present UK Government was repeatedly warned not to open.

To be fair to the Prime Minister he had little choice but to press on with Evel, or at least something like it. Not only was it a manifesto pledge, but its salience among Tory backbenchers and in the nation (England) at large demanded action. Something had to be done, and that something was Evel.

But as I've argued before, it's also the wrong answer to the right question - how is England to be governed? - but given David Cameron's obvious suspicion of grand constitutional projects it was always likely he'd go for the piecemeal constitutional option, the standard British approach if you like.

Indeed, last week ministers retreated in the face of a potential backbench rebellion and a barrage of criticism from former Scottish Secretary Alistair Carmichael, obviously keen to restore his political reputation, and "the 56", equally keen to demonstrate they have genuine clout in the House of Commons.

Since the referendum, Nationalists have rather shamelessly weaponised Evel in order to make life difficult for the Prime Minister despite having long supported it, as Alex Salmond recently admitted, "not just in principle but in practice". And although the SNP's U-turn was contrived with a hung Parliament in mind, the party realises it still has leverage given the Conservative Government's narrow majority.

But despite the usual opportunistic posturing, Nationalists have made some good points. First, as several have argued, the Government's attempt to implement Evel by stealth, via Commons standing orders, was shoddy and undemocratic, while as Nicola Sturgeon pointed out in a letter to Scottish Secretary David Mundell at the weekend, the procedure by which the Speaker will designate Bills as "England only" is as clear as mud.

As the SNP MP Pete Wishart (and new chairman of the Scottish Affairs Select Committee) wrote in The National last week: "English votes for English laws is something we don't necessarily have a problem with", having previously suggested a compromise under which the Speaker and Presiding Officers in Scotland, Wales and Northern Ireland would jointly designate England-only legislation.

After all, Evel doesn't just concern England, or even just England and Scotland, but every part of the UK, for once a precedent is set, it's difficult to see how Wales and Northern Ireland can escape its logic. Even within England there's trouble in store: as Greater Manchester gets more administrative autonomy, surely MPs from that region shouldn't be allowed to vote on health matters concerning other parts of England?

Meanwhile Evel, as Mr Wishart complained in his article, "could consign Scottish MPs to a second-class status", although whatever the final shape of the proposals it's difficult to see how that reality can be avoided. Past and present legislation has also illustrated how easily the SNP can tie itself in knots when it comes to its self-denying ordinance on apparently England-only Bills.

For example when, in early 2013, MPs voted on the Marriage (Same Sex Couples) Bill in the House of Commons, the SNP's then half dozen Members abstained on the basis it didn't apply in Scotland. Only certain clauses did have cross-border scope, giving rise to speculation at the time that the party was avoiding a potentially divisive issue (particularly for MPs representing more rural, and therefore more socially conservative, constituencies) ahead of the referendum.

Then there's this Wednesday's division on fox hunting in England, which has no funding implications for Scotland and is therefore unequivocally an England-only issue. Yet over the past few months both Angus Robertson, the SNP's Westminster group leader, and the First Minister have refused to rule out taking part in the vote, which, if passed by MPs, would bring English legislation into line with that of Scotland.

Although one can understand the party's temptation to help defeat a majority Tory government over such an emotive issue, the only honourable course of action is for its MPs to abstain. Indeed, if they decide not to, it could seriously undermine the more compelling aspects of their critique of the Government's proposals.

Evel, of course, won't apply in the House of Lords, another important (if largely unloved) aspect of the new Scottish constitution. Peers do not have geographical associations beyond their often-lyrical titles, although the SNP was still quick to identify a grievance when it became clear the UK Government planned to amend the Scotland Bill in the Upper House.

This, said Mr Wishart, would be an "absolute disgrace", although it's hardly a novelty for legislation to be amended by their Lordships; besides, any proposed changes would still need support from MPs in order to become law. Meanwhile, when a journalist with a long memory reminded Mr Wishart on Twitter he had long supported the SNP nominating peers (unlike his leader), he claimed to have changed his mind in light of the cash-for-honours scandal.

This was curious, for I clearly remember discussing the prospect of Nationalist peers with Mr Wishart relatively recently and he still seemed to support the idea. Anyhow it's an academic point, for such is the bile heaped upon their Lordships by Nationalists over the past few years that he probably realises it would be a U-turn too far, even for the SNP, even though its policy of boycotting one House of Parliament and not the other has never made much sense.

A few years ago there was talk of a very British compromise, with the Coalition keen to nominate the former Presiding Officer Sir George Reid as a member of the House of Lords. Usefully, he's no longer a member of the SNP, but for whatever reason (did Mr Salmond as First Minister veto it?) it didn't happen. The former MSP Jim Mather and former MP Alasdair Morgan were also spoken of as possible SNP peers, but again that plan fell by the wayside.

As Professor Page's new book points out, under the original Act of Union in 1707 Scotland sent just 16 "representative peers" (together with 45 MPs) to Westminster. All that's changed, of course, much like most other aspects of Scotland's ever-evolving constitution.