When you stopped people on the streets of Crawley a year ago and asked them what issue would determine their vote in the upcoming elections, one theme kept coming up.

Dozens of voters in three different locations – unprompted and vehemently – said immigration was the one thing above all others they cared most strongly about.

Back then, they knew what they were going to do about it: vote Ukip for the first time in their lives. And they did so in their droves. The European election result in Crawley saw Ukip top the poll with 32 per cent of the vote, while in council elections held the same day the party averaged around 20 per cent. But much can change in a year – and the extent of that change could help determine whether it is Labour or the Conservatives who get first chance to form a government after 7 May.

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Crawley, a new town built in the shadow of Gatwick airport, is one of those seats that Ukip cannot win at this election – but the size of the party’s support means it is almost certain to determine who will. In 2010, the Conservative candidate, Henry Smith, wrestled the seat from Labour with a majority of just under 6,000. It is a seat that Labour needs to win back again.

Much will depend on whether the voters (mainly former Tories) who switched to Ukip last year will return to the fold of the mainstream parties next Thursday.

Speaking to people a year on, it is quite clear that the issue of immigration has not lost its political potency. On a town centre shopping street it is only too easy to find voters who are angry and upset by what they see as an influx of immigrants putting unbearable pressure on services. Jill and Peter Smith are typical. “I just feel you go round Crawley and they’re taking over,” Ms Smith says. “We’re a minority now and I really do object to that. I think it’s got to be controlled – not stopped, controlled. I want the Conservatives to do it – not promise and talk about it but do it.”

In pictures: Experts' predictions for the General Election - 26/04/15 Show all 10 1 /10 In pictures: Experts' predictions for the General Election - 26/04/15 In pictures: Experts' predictions for the General Election - 26/04/15 Andrew Hawkins (ComRes) “This is an election of wild promises and political cross-dressing; so no differentiation, no breakthrough, no majority. The most likely outcome is Ed Miliband in No 10 because only he will be in a position to work with enough parties (that is, the SNP) to form a workable government.” Andrew Hawkins In pictures: Experts' predictions for the General Election - 26/04/15 Joe Twyman (YouGov) “As each day passes and the Conservatives fail to increase their vote share, the likelihood of Ed Miliband becoming PM (albeit still with a minority) grows. That’s due to the continued performance of the SNP in Scotland and former Lib Dem voters propping up Labour in England.” In pictures: Experts' predictions for the General Election - 26/04/15 Ben Page (Ipsos MORI) “A better week for Ed Miliband than for David Cameron – personal ratings improving, and the betting markets moved in his favour. The poll numbers have him ahead by a whisker which, combined with the electoral maths, makes him more likely to be able to form some kind of government on the raw numbers right now. However, Cameron will be hoping for a 1992 moment when ultimately voters can’t quite bring themselves to try someone else – and 38 per cent of voters still say they may change their mind.” In pictures: Experts' predictions for the General Election - 26/04/15 Rick Nye (Populus) “The Populus/Hanover Predictor has Ed Miliband ending up as Prime Minister in more than five out of six simulations we run.” In pictures: Experts' predictions for the General Election - 26/04/15 Nick Moon (GfK) “Tories most seats, SNP 45 or more, Ukip fewer than five: minority Labour government.” In pictures: Experts' predictions for the General Election - 26/04/15 Damian Lyons Lowe (Survation) “Ukip’s vote share is prone to be squeezed, benefiting the Conservatives, while the SNP’s onslaught on Labour continues. Based on three UK/Scotland polls, I am upgrading the Conservatives and the SNP, and downgrading Labour. Both the Lib Dems on 30 seats and Ukip on 5 to 10 remain unchanged.” In pictures: Experts' predictions for the General Election - 26/04/15 Michelle Harrison (TNS) “Still no change, but Nicola Sturgeon goes from strength to strength in Scotland. For me, who the biggest party is will hinge on the crucial marginals. But what ultimately wins – safety first, for the Tories, or Labour’s ‘get out the vote’ ground war?” In pictures: Experts' predictions for the General Election - 26/04/15 James Endersby (Opinium Research) “Our poll today shows a one-point Tory lead, statistically a tie. It’s going to be incredibly tight. Labour and the Tories would do well to be negotiating with the smaller parties behind the scenes. In the likely event of a hung parliament, we’ve been testing various combinations of coalition. Labour teaming up with the Lib Dems would be more popular than with the SNP, which in turn would be more popular than a Tory/UKIP scenario.” In pictures: Experts' predictions for the General Election - 26/04/15 Martin Boon (ICM) “Not much going on really, and no reason to change my mind. Tories by a couple of points. In pictures: Experts' predictions for the General Election - 26/04/15 Lord Ashcroft (Lord Ashcroft Polls) He refuses to make predictions. “My polls are snapshots, not predictions.” Rex

Yet people are less certain to vote Ukip than they once were, and the issue of Scotland has also come to the fore.

“I don’t want Nicola Sturgeon to get anywhere near – anywhere near – Downing Street,” says Ms Smith. “If there is anybody I dislike most it is her. She makes my hair stand on end. Political correctness – forget it.”

Another voter, who gives his name as Gary, agrees – but he will be sticking with Ukip. “I think Ukip’s doing the job,” he says. “People say there is no chance of them getting in so really you’re wasting your vote. I don’t think so – if they get more votes they’ll get elected.”

But his wife, who declines to be named, says she will switch to the Conservatives – on the basis of the SNP threat. “What I don’t want is for Scotland to be having a say. They wanted to leave and they didn’t, so now they want to poke us in the eye. Get back the other side of the border.”