Ed Miliband emphatically ruled out a coalition or a deal with the SNP last night.

Does that mean he’s ruling himself out as prime minister?

Was he on course to win anyway?

Who’s going to win the election?

Obviously the simple answer is that no one knows, but we have a wealth of polling that gives us some idea of how people are likely to vote on 7 May.

The Press Association’s latest “poll of polls” (a rolling average of polls from the last seven days) has the Conservatives on 34 per cent, Labour on 33.3 per cent, Ukip third on 13.2 per cent, the Lib Dems on 8.5 per cent and the Greens on 5.2 per cent.

Other “polls of polls” are available but as of today, the results are similar: the Conservatives have a small lead.

Now we have to feed this into some kind of model which predicts how those levels of support will translate into actual parliamentary seats.

These two things will be different, because Britain’s first-past-the-post system disadvantages some parties.

Despite the fact that Nigel Farage’s party is ahead of the Lib Dems in the polls, most experts think it unlikely that Ukip can win more than a handful of seats, because its vote is not spread over a wide enough geographical area.

There is a range of different prediction websites available, all using different methodologies. We’re going to use www.electionforecast.co.uk because its predictions are pretty central. Here’s the latest:

The Conservatives are predicted to win more seats than any other party, but they won’t be able to govern alone with 280 seats. Everyone agrees that there will be a hung parliament. 

You need 326 seats for a Commons majority, which guarantees the government will win votes in parliament (or 323 if, as usual, Sinn Fein don’t take their seats).

Conservatives plus Lib Dems only gives us 307 seats, so there won’t be a repeat of the current coalition.

The Conservatives and the SNP together would have a majority, but Nicola Sturgeon has ruled out a coalition or a deal with the Tories.

So it’s Labour/SNP?

That seems the most likely option, although those two parties would not have enough seats for a majority either on this model. A Labour/SNP government would need the support of nine more MPs.

But hang on – Ed Miliband is saying: “I’m not going to have deals with the SNP or coalitions.”

That appears to rule out both a proper Labour/SNP coalition and a less formal “confidence and supply” deal, in which a smaller party agrees to help support a minority government in votes of no confidence in return for some kind of policy concessions.

So Ed Miliband won’t be prime minister?

Wait – he could be. There’s a fair bit of doubt about what Ed Miliband actually means when he says he won’t do a deal with the SNP.

Labour appears to think it will be politically impossible for the SNP not to vote for a Labour government when it comes to a crunch vote.

Even if there is no deal, the SNP could not bring down a Labour government and bring in a Conservative one without making itself massively unpopular with its own supporters. Or that is the political calculation.

What would happen in practice is that Labour would form a minority government and would have to canvass support from the SNP or indeed other parties every time they wanted to pass a piece of legislation.

Could a minority government survive?

It’s possible Labour could govern as a minority party, although history is not on its side.

Jim Callaghan’s Labour minority government – chiefly remembered for the strikes of the Winter of Discontent – lasted from 1976 to 1979 before being brought down by a vote of no confidence.

On the other hand, Alex Salmond led an SNP minority government for a full term at Holyrood between 2007 and 2011.

Some political scientists, like Professor Colin Talbot from the University of Manchester, think the Fixed Term Parliaments Act of 2011 has made it dramatically easier for minority governments to survive.

Now a government can lose a key vote like the budget, but it cannot be forced from office.

Under the act, the government only falls if it decides to resign, or if it loses a vote of no confidence. Unless an alternative government wins a confidence vote within 14 days, there is a general election.

So even if a minority government loses a no-confidence vote it has two weeks to cobble together some other kind of power-sharing deal that will enable the dominant party to hang on to power after all.

How will things actually play out on 8 May?

Let’s assume the Conservatives win the most seats but don’t have enough for a majority or another coalition with the Lib Dems. Two things could happen:

1) David Cameron could accept that he won’t be able to get a working majority and resign, which means the Queen will call on Ed Miliband to try to form a government.

This makes things easy for Labour. They automatically take the reins without having to win a confidence vote.

2) Or David Cameron could try to stay on and form a minority government himself.

His first key vote would be the Queen’s speech. Traditionally, if the government lost this it would be expected to resign.

But under the Fixed Term Parliaments Act it doesn’t have to – the only vote that can force the government out is a motion “that this house has no confidence in Her Majesty’s government”.

There will be another election unless an alternative government drums up a majority in another vote “that this House has confidence in Her Majesty’s government” within 14 days.

This makes things more difficult for Labour, who first have to get enough support in the Commons for a no-confidence motion to oust the Conservatives, then form a new government, then win a confidence vote themselves.

As Dr Chris Hanretty from the University of East Anglia points out, the way things play out has implications for the balance of power.

If Mr Cameron forces the Commons to vote his government out of office, the SNP would be forced to explicitly vote for a Labour government, which both Labour and the Conservatives could use against the nationalists.

Prof Talbot says the SNP won’t like being forced to support Labour without policy concessions, but they “are pledged to ‘lock out’ the Tories and are unlikely to want to force a new general election, which is what would happen if Labour failed to win the vote”.

“So it could come down to a ‘game of chicken’ as to who blinks first – Labour or the SNP.”