William Hague moves backwards, holding a bag of potatoes. A scrum of TV cameramen surges up the Shettleston Road towards him. “How much have the tatties gone up?” demands a reporter. “Well, that's more a matter for Annabel, obviously,” says Mr Hague inelegantly, glancing sideways at the ample figure of the Scottish Conservative leader, Annabel Goldie. “But it's a lot.”

The cameras roll, the spin-doctors make worried notes, the shoppers sneak by. So close is the call in this Glasgow East by-election, so talismanic the seat, that even a Tory ex-leader's views on a trolley-load of groceries could, we sense, swing the entire result and influence the fate of a Labour Government.

Inside, as the pack moves down the supermarket aisles, the nature of the contest takes tangible form. Rachel McGarrigle, a pensioner and lifelong Labour supporter, hesitates in front of the mushrooms. She confesses that, for the first time in her life, she has become a swing voter. She is unhappy about Labour's broken promises, the increase in immigrants, and the dreadful potholes in the road outside her house. But most of all she worries about the price of fuel. “Why are we paying so much, when Scotland has so much oil of its own?” she asks.

I put her down as a Scottish National Party convert. Luckily I have already interviewed the SNP leader, Alex Salmond, so have no need to relay this story to him: his glee would be almost unbearable.

As it is, he is beginning to exhibit signs of overweening optimism. A solid Labour majority of 13,500, an unimaginable obstacle in normal times, could, he claims, melt away. His argument is based on anger and hope: anger at the rise in the cost of household budgets, and hope in the promises held out by the Nationalists.

Glasgow East is not, however, a place which majors on hope. The terrible health statistics of its worst areas tell a story of years of deprivation and low expectations.

Mr Salmond looks elsewhere for the answers - to an air of self-reliance and self-improvement he claims to detect in other parts of the constituency. It is true that there are far more middle-class enclaves in Glasgow East than the popular view suggests. In the past these have been solid for Labour, and it is here that the true test of traditional loyalties lie. Margaret Curran, the Labour candidate, is out in the Garrowhill area, receiving a respectful, if muted, hearing.

“Any pensioner who doesn't vote Labour needs their head examined,” says Tom Mahon, now retired. “I've voted Labour all my life, and I've no intention of changing now.” A tick goes in the canvasser's box. But just down the road Jim McGrory, 48, is not so sure. He cites Tony Blair, Iraq and Afghanistan as three reasons for switching.

To fight this, Ms Curran believes that her reputation as a no-nonsense Glaswegian and an experienced minister at Holyrood will stand her in good stead. “I'm struck by how open and enthusiastic people are,” she says.

But she is fighting on many fronts. Since the SNP took power at Holyrood, Labour has been on the rack in Scotland, seeing its leader, Wendy Alexander, driven out of office.

The SNP candidate, John Mason, a mild-mannered accountant and local councillor, has sought to exploit this by claiming that he can see no difference between Gordon Brown and Margaret Thatcher, his point being that both served Glasgow badly.

“That is simply offensive to hundreds of Labour supporters,” Ms Curran says. She points to the huge investment made in the area under Labour, the new schools, new housing and the further education college.

Against this, the SNP appeal is intangible. It offers heady dreams of independence, which Mr Mason plays up, but which Mr Salmond rarely mentions. He talks instead about the economy, investing in small businesses, and fuel regulation.

The Tories have one other asset - a dream candidate in Davena Rankin. Glasgow-born and bred, half-Scottish, half-Caribbean, mother to a five-year-old, and a branch secretary for Unison, she is the kind of new Tory that David Cameron would struggle to invent. She has, of course, no chance; but, as she and Mr Hague progress down the Shettleston Road, I encounter an unlikely convert to the cause.

Richard Baxter, 56, a former shipyard worker and friend of Jimmy Reid, hero of the Upper Clyde work-in in the early 1970s, is inveighing against Labour. He tells me he has met Glasgow's only Tory councillor, David Meikle, and was rather taken with him. Ms Rankin has just drifted by, and he is quite taken with her as well. This socialist just might vote Tory.

The battle for third place is almost as ferocious as the SNP-Labour tussle for first. The Liberal Democrats, who pushed the Tories into fourth place last time, have a strong and articulate candidate in Ian Robertson, a teacher, local man, and already tipped for great things in the future. For now, the betting is that Labour will hold the seat with a severely reduced majority. All will depend on who is best at turning out the vote on the night - there are no certainties here any more.

That is not the only surprise that this by-election yields. I am talking to Mr Salmond after watching teenagers put on a spirited performance in song and speech, based on the theme of national identity.

Mr Salmond comments on the self-confidence that performing on stage can give. “We want more of that in Scotland,” he says. “I sometimes wonder how it is that the English public school system seems able to instil that sense of superior self-confidence in its pupils, whatever their ability. We could do with some of that in Scotland.”

When a Scottish Nationalist talks up the English public school system, the Labour Party struggles to find its soul, and a half-Caribbean trade unionist carries the Tory flag, then anything, one senses, could happen.

By-election betting

More bets have been taken on the Glasgow East poll than on any other by-election in history, according to Ladbrokes

Labour is the 4-11 favourite, with the SNP at 15-8

Nick Weinberg, spokesman for Ladbrokes, said: “Labour attracted the vast majority of early money, then the tide appeared to be turning to the SNP last week, but that’s subsided and Labour are once more who the punters want to be with.

“The unprecedented interest is a product of the current political climate. At any other time, Glasgow East would be regardedas a relatively safe Labour seat and we would expect the market to reflect that”

Industry-wide, the amount wagered on Glasgow East is £250,000 so far, most of it online. That compares with a total of just over £100,000 for the by-election in Crewe & Nantwich in May

Ladbrokes is also taking bets on the turnout. The 5-2 favourite is currently somewhere between 35 per cent and 39.99 per cent

Source: Ladbrokes

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