One of the less anticipated consequences of the recent televised leaders' debate was the mainstreaming of Welsh and Scottish nationalism across the UK.

On the night itself one of the most googled queries was "Can I vote for the SNP?" And although that might sound like a straightforward enough question, most of those asking it were in England.

This is not, as it happens, as wild a scenario as it might first appear. In the general election of 1885 a journalist and biographer called T. P. O'Connor was returned both for Galway and the "Scotland Division" of Liverpool.

Now O'Connor was a Parnellite Home Ruler, but he chose to sit for Liverpool, which had a large Irish population, thus his election as an Irish Nationalist, and, remarkably, he represented the same constituency until his death in 1929, winning four elections after his party had achieved the independence of (most of) Ireland from the UK.

But despite what she recently called "temptations and encouragement" - a YouGov survey suggested the SNP would do better than the Liberal Democrats in England - Scotland's First Minister has no plans to follow O'Connor's example and field candidates in, say, Corby, while Plaid Cymru is also concentrating on home turf.

In a recent interview Leanne Wood, Plaid's leader since 2012, confessed to being staggered at the response to her performance in the debate, not least her telling UKIP leader Nigel Farage he ought to be "ashamed" of himself. People, she said, kept telling her they wished "they had a party like Plaid Cymru to vote for in England".

I chatted to Ms Wood on Saturday evening at a gala dinner in Cardiff to celebrate the 90th birthday of "the Party of Wales", and she was everything I remembered her being when I interviewed her a few years ago: engaging, well-briefed on policy detail and refreshingly un-politician like. As one of her advisers remarked, the leaders' debate meant that everyone in Wales (and indeed beyond) now knew it too.

And Plaid Cymru, to be blunt, needs all the good news it can get. Recently it's been polling behind UKIP (which could secure as many as five Assembly Members at next year's devolved elections) and one opinion poll put support for independence at just three per cent. For obvious reasons, many Welsh Nationalists eyes glaze with a mixture of awe and jealousy at any discussion of the SNP "surge" north of the border.

But then Wales, of course, is not Scotland, although the Plaid leader is at once familiar to any observer of Scottish politics. Read any profile of Leanne Wood and you'll see that she joined her party at a young age (20), driven by hatred of Trident and tribal disdain for Labour, while more recently she - like Ms Sturgeon - has worked hard to make the politics of nationalism and social justice almost synonymous. Even sartorially the pair increasingly resemble one another (Leanne was supposed to don red shoes tattooed with the Welsh dragon at the leaders' debate but thought better of it).

Why, then, is Plaid not even the biggest opposition party in the Welsh Assembly (that position is filled by the Conservatives, less toxic than they are in Scotland)? This is especially curious given how well the party originally did out of devolution: when I was a journalism student in Cardiff 16 years ago Plaid had recently taken hitherto Labour strongholds in the Welsh Valleys at the first Assembly elections - almost a decade before the SNP made similar inroads in urban Scotland.

The small 'n' nationalism of the Welsh Labour Party goes some way to answering the question. Carwyn Jones, the current First Minister, has high approval ratings and is generally seen, like his party, to "stand up for Wales". And while Plaid, under Ms Wood's leadership, has tacked to the Left in an attempt to compete (its membership card still carries the word "socialist") it poses little serious electoral threat to the party of Bevan and Kinnock.

Constitutional politics also provides part of the explanation. Since 1999 the three Unionist parties in Wales (even the Conservatives) have taken ownership of the "more powers" agenda, thus depriving Plaid of a unique selling point. Even a majority of Plaid voters, meanwhile, don't want independence, and indeed the 'i' word is seldom mentioned. As Ms Wood put it in yesterday's Sunday Herald, the goal is "greater self-government" and "ultimately" independence.

Independence for Wales (whose economy is smaller than that of Greater Manchester) simply isn't perceived as economically credible, which hasn't been the case (rightly or wrongly) in Scotland for about a decade, and strategists see building economic confidence as crucial to increasing support for, at first, Plaid, and then what Plaid's founding fathers referred 90 years ago as "the greatest measure of Home Rule".

It won't be easy, but judging by the mood in Cardiff Bay on Saturday night the current generation of Plaid leadership are in remarkably high spirits, believing that Leanne Wood's newly-boosted profile will contribute to (relative) success in the forthcoming general election and then - in what would constitute a major electoral shift - a breakthrough in next year's Assembly elections.

Plaid is currently being coached to think positively by the shrewd and energetic Claire Howell of the London-based firm RedCo ("The Really Effective Development Company"), who's hoping the successful role she had with the SNP in the last two Scottish Parliament elections (and the referendum campaign) will also work in Wales. Key to it all is Leanne Wood, but crucially Plaid are starting from a much weaker base than the SNP even at its lowest ebb a decade ago.

The Scottish First Minister has made much of a potential "progressive alliance" of three female party leaders (the other being the Greens' Natalie Bennett), and there's an obvious affinity between Ms Sturgeon and Ms Wood: they are said to keep in touch via WhatsApp while the Plaid leader was given star billing at the SNP's 2014 autumn conference.

But this camaraderie masks significant differences: on the Barnett Formula Plaid and the SNP are at odds for the simple reason that Wales loses out while Scotland gains, and beyond simplistic "anti-austerity" rhetoric Plaid is well aware that its heart beats significantly to the left of their triangulated Scottish cousins. And while firmly anti-Trident Ms Wood is (rightly) cautious about "red lines" ahead of 7 May. "To go into talks in terms of where you won't negotiate", she recently reflected, "is generally not helpful."

Nuance, however, is unlikely to feature on Thursday evening when the Plaid and SNP leaders joust with three other "opposition" parties on network television. In Wales politics still resembles that in England: two main parties and nationalism a marginal force; Plaid could easily go into this election with three MPs and come out with the same number. Leaving Cardiff yesterday morning it was tempting to conclude that the Anglo-Welsh Union of the mid-16th century remains relatively secure - unlike its Anglo-Scottish equivalent.