Jane Collins insists the timing was "purely coincidental". Yes, said Ukip's candidate for Thursday's Rotherham byelection, it was something of a boon that the party had received so much publicity after it emerged that Rotherham council had removed three young children from their foster parents because the couple were members of her party. But it was not a classic case of cynical news management. "Obviously we're getting insinuations now that this is something we've made public to hijack the byelection and that absolutely is not true," she said on Sunday.

The couple in question simply called the Yorkshire branch last week "at the end of their tether", said Collins. "They phoned our branch secretary and asked what on earth they could do to get the children back."

Collins went to their house and by Friday the couple were on the front page of the Telegraph. By Saturday both Labour and the Conservatives had condemned the Labour-run council, saying membership of a "mainstream" political party should be no bar to fostering.

It was a golden moment for Ukip, and leader Nigel Farage knew it. "I'm delighted that David Cameron no longer thinks I'm a closet racist," he told ITN, taking the opportunity to claim "the game was up" for those who thought that calling for an end to Britain's "open door" immigration policy was fundamentally racist.

Ukip was already heading for a good result in Rotherham, buoyed by its third place in the Corby byelection earlier this month. Collins went one better in last year's Barnsley byelection, when she came second to Labour, and, with polls putting national support for Ukip at 7%-9%, is cautiously optimistic.

You can't buy the sort of publicity the foster scandal has brought the party, but serious money is being poured into the campaign nonetheless. The party bussed in activists from all over the country for a "day of action" on Saturday, which featured Farage touring the town's pubs preaching his anti-Europe, anti-immigration gospel.

The message is getting across – a traffic warden, who said she could not be named because she was employed by the council, admitted she had voted Ukip in the recent police and crime commissioner election. "And as I did I looked up and told my dad I was sorry," she said. "He was a lifelong Labour supporter and would be turning in his grave." She still thought Labour would win. "You could put a red rosette on a pig and they'd vote for it in Rotherham."

It was a phrase rolled out time and again all over the South Yorkshire town, sometimes with a donkey or a monkey substituted for the pig.

A glance at Rotherham's voting history explains the cynicism. Almost 80 unbroken years of Labour, ending with an 18-year stint enjoyed by Denis MacShane, the former Europe minister who resigned in disgrace after fiddling his expenses. Labour is pulling out all the stops to keep the seat, but success is by no means assured – especially with an unusual number of fringe candidates who can reasonably expect not just to keep their deposits and eat into Labour's 10,000 majority but quite possibly outperform the Tories and Lib Dems too. The rise of minor parties is a national trend: a Guardian/ICM poll earlier this week put support for "others" (ie not the big three) at 15%.

Ukip is not the only party capitalising on ill feeling towards immigration in Rotherham. Sitting eating a sausage roll outside Jamie Oliver's Ministry of Food cookery school, Sarah was surprised to hear there was a byelection on. She didn't vote, she said – "They always seem to hold them on days when I'm working" – but she knew who she would like to see representing Rotherham at Westminster. "I think the BNP should get a chance."

"I'm not racist," said the 32-year-old single mother of two, "but I don't think we'd be in that much debt if we didn't have all these foreigners. We're living in poverty and they get their bills paid, they get Sky, mobile phones. I have to watch my money."

She was aware that the BNP was considered hateful by many – that is why she would not give her surname – but thought that if the far-right party were in power, "we might be able to live in a civilised country."

Nick Griffin's party polled 10.4% of the Rotherham vote in the 2010 general election. Immigration is a big issue in the town, which has been hit hard by the recession. On Friday the Indian-owned Tata Steel announced that 110 more jobs were to go from the constituency, which has become a popular place for eastern Europeans to settle since EU enlargement in 2004. When the 2001 census took place, just 4.3% of the constituency were born outside the UK, with 5.4% of respondents (about 4,700) saying they were Muslim. Local estimates suggest both percentages have probably now doubled as a result of EU enlargement and natural growth.

Alongside the BNP, the English Defence League is fielding a candidate, Clint Bristow, as an independent. According to the anti-racism group Hope Not Hate, which is campaigning hard in Rotherham, Bristow is a "violent thug" with a criminal record for threatening Muslims.

The English Democrats also fancy their chances, having come second in this month's PCC poll in South Yorkshire. Over half a pint of Sneck Lifter local ale, their byelection contender, David Wildgoose, explained why he was confident of at least retaining his deposit.

People were fed up with unfettered immigration, he said, and "unlike Ukip, who only appeal to disgruntled Tories" – of whom there are relatively few in Rotherham: they won just 16.7% of the vote in 2010 – "we are appealing to disgruntled Labour voters too". Most people agreed with him, he said, that multiculturalism in Rotherham had failed, "because it emphasises differences rather than celebrating what people have in common".

But it is the Ukip campaign that appears to have the most momentum. Collins – who used to be an aide to Ukip's MEP for Yorkshire & North Lincolnshire, Godfrey Bloom – felt she had a headstart. As Ukip's Yorkshire regional organiser, she is always on the stump: "We don't start campaigning when there is an election on. People see us all the time."

Keen to prove that her party appeals not only to white Britons, Collins has roped in two Asian restaurateurs, Amjad and Mujeeb, to help. "They can speak Urdu," she said.

On the doorstep, she emphasised her grassroots credentials. Knocking on a door just around the corner from MacShane's constituency house – where he claimed £125,000 over seven years for an office that turned out to be his garage – she asked 74-year-old David Evans what he thought of MacShane. "I daren't say what I think of him – I'm a Labour voter," said Evans. "He's a waste of space, isn't he? But he's not alone." He knew of Ukip, having seen the party leader on Have I Got News for You? the previous week. "What d'you call him? Le Fage? I saw him on the telly. He should be a comedian."

On the street outside, Robert Miles, an out of work locksmith, complained that he was finding it impossible to get a job. "You'll probably agree, then, that this government's open door immigration policy is wrong?" asked Collins hopefully. "To be honest, I just want a job," said Miles.

So who will win? Labour are certainly nervous. Sadiq Khan, from Tooting, who was one of the army of MPs ordered up to Rotherham to help this week, said he thought it was a "turnout" election – "whoever can get their vote out will win".

The weather was a problem, he said, and the early nights. But he felt the sheer number of candidates on the ballot papers – 11 – would be a bonus for Labour because it would split the protest vote. "In Bradford West, if you weren't happy with Labour or the other main parties and wanted to register a protest vote, you went for Respect. This time there isn't one obvious repository for those sort of votes."