The final session of Prime Minister’s Questions revealed that one of the themes of this election campaign is going to be attempts to scare us about the tax plans of the major parties.

Ed Miliband used the final question time before Parliament breaks up for the May 7 general election to ask the Prime Minister whether a future Conservative government would raise VAT.

Mr Cameron replied by comprehensively saying it wouldn’t – but this won’t stop Labour, which insists that Mr Cameron cannot be trusted and VAT will go up if the Tories win the election.

The Conservative leader returned the favour by asking Mr Miliband whether Labour would raise National Insurance – which the Conservatives call a jobs tax.

Mr Miliband couldn’t or wouldn’t answer, which looked terrible. The session was a car crash for the Labour leader.

However, within an hour the shadow chancellor, Ed Balls, was telling journalists that Labour’s manifesto would include a commitment not to increase VAT.

So that answers that.

Except that it won’t stop the Conservatives claiming that Labour actually has secret plans to increase National Insurance.

Conservative chairman Grant Shapps insisted: “It’s becoming clearer by the day that Ed Miliband has a secret plan for a new jobs tax and other higher taxes on hardworking people. Labour did it before and they’ll do it again.”

Meanwhile, Mr Balls spoke to regional newspapers including the Birmingham Post in Westminster, telling them that there was no way the Conservatives could cut the deficit as quickly as they say they plan unless they raise VAT or cut NHS spending.

It was simply impossible to make the required savings by making even more cuts to police or local councils, said Mr Balls.

“Unless David Cameron and George Osborne are going to fight the election confirming they will have decimatingly deep cuts to police and defence and social care and other public services which every one knows are undeliverable, I think they are going to have to admit they are going to ut the National Health Service or raise VAT,” he said.

Secret plans for tax rises are one scare story we’ll hear a lot of during this election.

Another is that idea that one or other of the major parties will have to do a deal with an unpopular smaller party.

In the case of the Tories, the claim is that they could end up doing a deal with UKIP – which is of course very popular with some people, but considered somewhat extreme by others.

However, it remains to be seen whether UKIP really will win a significant number of seats. Anything can happen on May 7 but opinion polls, which can of course be wrong, suggest UKIP is unlikely to come first in many constituencies even if it may came a strong second or third in many parts of the country.

What is perhaps a little more credible is the idea that the SNP will win a significant number of seats in Scotland.

This could be anything from 25 seats to as many as 50, depending on who you listen to. But they will be a force to be reckoned with.

By comparison, the Lib Dems won 57 seats in 2010.

Is it possible that Labour could be the largest party, but without a majority, after May 7? Of course.

And the SNP could therefore hold the balance of power.

SNP politician Alex Salmond has been touring the television studios recently to boast about the influence his party could have.

He claimed the SNP would order Labour to end “austerity”, effectively forcing Labour to tear up its entire economic strategy.

Labour is committed to making further cuts, on top of the cuts already imposed by the Coalition (and it’s worth remembering that even accepting the existing cuts is not mandatory – governments can also increase spending on services such as police and local councils if they wish, assuming they can think of a way to foot the bill).

Labour plans to make smaller cuts than the Conservatives, and we could debate whether “austerity” is the right word, but Mr Salmond wants them to change course.

He also says he would demand that construction of the high speed rail line between London and Birmingham begins in Scotland, something which would delay the project for a decade if he really means it.

Despite being a former SNP leader and Scottish First Minister, Mr Salmond is a backbench member of the Scottish Parliament – not the leader of his party, nor leader of the SNP group at Westminster.

He’s in no position to set out the SNP’s terms for working with Labour.

And there is no chance that Labour will allow the SNP to dictate their plans, or not to anything like the extent Mr Salmond suggests. They’d rather have a second election than that.

But Mr Salmond’s comments are mana from heaven for the Conservatives – who will attempt to terrify English voters with visions of a man who wants to break up the UK running the country.