Lord Ashcroft has been teasing the political world by suggesting that his polling of Scottish marginals will confirm what Labour fears – that a surge in SNP support will reduce the number of Labour MPs north of the border from 41 to – well, possibly just four. A wipeout.

But before we get carried away with the implications of the Scots’ new-found love of the SNP, let's not forget England. It’s south of the border where most UK voters live - and where there could be an even bigger upset than that threatened by the Scot Nats.

More than four out of five UK voters live in England. Out of the total number of 650 seats represented at Westminster, 533 are in England. And included in that number are the vast majority of the 100 battleground marginal seats where this election is really being fought out.

Ashcroft will clearly time the release of his Scottish polling for the maximum exposure – probably this Sunday. And who can blame him when he’s spent hundreds of thousands on his marginal polls?

But equally interesting – and possibly just as good a pointer to what’s going to happen on 7 May - are the national voter intention polls Ashcroft produces every week. Why? Because they include a very telling regional breakdown.

In the latest Ashcroft national poll, the Tories and Labour are tied on 32 per cent. But look at the national breakdown: in England, Labour lead the Conservatives by 35 per cent to 31 per cent. That is a hugely significant swing to Labour since 2010 when David Cameron’s Tories swept all before them, leading Labour by 39 per cent to 28 per cent.

Mike Smithson of Political Betting has been arguing for some time that the SNP surge north of the border, first identified by polls in November, is masking a Tory collapse in England.

This week, to emphasise just how serious this could be for Cameron, Smithson recalls what happened in the 2005 election – the third of Tony Blair’s New Labour victories.

The Tories won 35.7 per cent of UK voters’ support – which got them 194 seats. Labour achieved only 35.5 per cent – but ended up with 286 seats.

The big question is: could it happen again on 7 May? Smithson says one of the great drivers in recent elections has been anti-Tory tactical voting – “those opposed to the Tories being much more ready to switch their votes than those opposed to Labour”.

This helped both Lib Dems and Labour in their separate battles with the Conservatives. “My guess, says Smithson, “is that we’ll see a repeat of the pattern from previous elections.”

In 2010 it was a strong performance in Scotland that kept Labour from complete disaster. That clearly isn’t going to happen this time: but a strong performance in England – even a Tory collapse on this side of the border – is a real possibility.