Voters go to the polls today to choose new MPs for the constituencies of Clacton and Heywood & Middleton.

For anyone planning on staying up, the result from Clacton is expected around 2.30am, while Heywood should be around 3am.

The winner of both contests is a foregone conclusion. Douglas Carswell will retain his seat of Clacton, albeit as a member of Ukip rather than the Conservative party; and Liz McInnes will retain Heywood & Middleton for Labour.

But this doesn’t mean the results won’t be significant – far from it. Here are five things to watch for.

1) The size of the victory

Ukip won’t just want to win in Clacton, they’ll want to win big. There have been two opinion polls carried out in the seat, but both date from over a month ago. One gave Carswell a 32-point lead; the other suggested it could be 44 points. This has obviously raised expectations of a huge victory for Ukip, which in turn has put the pressure on the party to deliver just such an outcome. A win of anything under 20 points would therefore be a bit of a comedown, though it would set the scene nicely for a close-run contest in Rochester & Strood.

Labour has been predicted a similarly comfortable win in Heywood & Middleton. Again, there have been two opinion polls conducted in the constituency; both found leads of 19 points. A margin of victory over Ukip of anything under 10 points would therefore count as a something of an upset.

2. The share of the votes

Douglas Carswell won 53% of the vote in Clacton in 2010 when he stood as a Conservative. Will he do even better this time? The two opinion polls from the campaign put him on 64% and 56% – but to repeat, they were both conducted over a month ago. We’ve had no clue as to what may or may not have happened since.

Jim Dobbin won 40% of the vote in Heywood & Middleton in 2010. There’s a good chance Liz McInnes will do even better this time, thanks to the defection of a big chunk of the 2010 Lib Dem vote. If Labour gets near 50% the party will have cause to celebrate. But as Harry Lambert points out on the New Statesman’s election website, Labour’s share of the overall electorate in the constituency has all but halved since 1997. The greater its share in today’s poll, the greater McInnes’ legitimacy as an elected member of parliament.

3. The runners-up

Labour came second in Clacton in 2010, polling 25% of the vote. Ukip didn’t even stand. The Tories will be hoping they manage a respectable second place this time, though a share of the vote below 25% would be something of a humiliation. Labour will do well to secure third place with double figures.

In Heywood & Middleton, the Tories came second in 2010 with 27% of the vote. Ukip were fifth on 2.6%, behind the Lib Dems and the BNP. Quite clearly things will be different this time. A lot of attention will be focused – justifiably – on how strong a second place Ukip can manage. The party has bragged of giving Labour a run for its money. Will this turn out to be bluster or fact?

4. The turnout

Might the added spice of a Ukip insurgency boost turnout in Clacton above levels seen in recent by-elections? Somewhat uncannily, the turnout in the two most recent contests where Ukip was a strong challenger – Newark in June 2014 and Eastleigh in February 2013 – was exactly the same: 52.8%. The figure for Clacton in 2010 was 64.2%. I’d say somewhere between 50 and 60% would count as a good turnout today. Anything below 50% and the whole affair can be said to have failed to significantly enthuse the electorate.

The turnout in Heywood & Middleton in 2010 was 57.5%. Don’t expect anything near this level today. In the two most recent by-elections won by Labour, turnout was pathetic: 28.2% in Wythenshawe & Sale East in February 2014 and 39.3% in South Shields in May 2013. Anything below 30% today would be pretty rotten.

5. The Lib Dems

Before today, the Liberal Democrats had contested 16 of the 18 by-elections to have taken place in this parliament. They lost their deposit nine times. As of today, the total number of by-elections in this parliament rises to 20; and as of tonight, the number of lost Lib Dem deposits is likely to rise as well. In 2010 the Lib Dems won 12.9% in Clacton and 22.7% in Heywood & Middleton. Watch to see if the party ends up plummeting below 5% in both contests. If they do, they’ll forfeit two deposits and be £1,000 the poorer.