“You were only supposed to blow the bloody doors off!” Michael Caine's famous line from the Italian Job echoed in my ears when reading Labour's 2019 election manifesto. The document will be celebrated by socialists for many years, but it may have blown what chance Jeremy Corbyn had of winning the general election.

Labour's manifesto involves £83billion extra spending per year, plus a £400bn transition fund over the next five years. Two weeks ago the Tories were widely criticised for estimating that Labour's plans could cost over a £1.2trillion all told. However, that may not be far off.

Spending on a million council homes, increasing public sector pay, free broadband, green transformation etc amounts to £800bn, according to the manifesto. But this is before counting the cost of six major industry nationalisations, the four-day week and freezing the state pension age.

I found myself agreeing, policy by policy, with a lot of what Labour is calling for. Rail nationalisation, check. Windfall profits tax, check. A financial transactions tax to curb reckless speculation, check.

But the problem is the sheer volume of spending pledges. By Labour's own figures, this is twice as costly as their much-praised 2017 manifesto. Labour haven't just thrown the kitchen sink at this election; they've ripped up the floorboards and walls, and bet the house on the biggest peacetime spending programme in history.

Nicola Sturgeon must be tempted to say "bring it on" because, if Labour's plan ever happened, it would pump around £100bn into Scotland through the Barnett Formula. But would it never happen? There's little sense in offering election giveaways that people don't believe in.

The BBC Question Time audience is not representative of the electorate, but the reaction to Labour's plans was instructive. “Back to the 70s...flight of capital...trades union barons...financial crisis”. Just the words the Conservatives want to hear. They will no doubt appear in bogus Tory “fact check” posts from now till election day.

The Institute for Fiscal Studies actually agreed with the BBCQT ranters. Indeed, their director, Paul Johnson, was even more scathing. “Simply not credible”... “impossible to achieve”... “totally unrealistic” were just some of his comments on Labour's economic prospectus. The IFS is the gold standard on public spending.

Johnson claimed that Labour would have to do much more more than just tax the top 5% of earners and increase corporation tax. The new taxes on earnings over £80,000 would only yield about £6bn. If the entire burden was placed on firms, many would simply leave the country.

To have Scandinavian levels of state provision, the UK would require Scandinavian levels of taxation – on all of us. That may be a good thing, but it needs to be done judiciously.

Now, the Scottish government has already implemented many of Labour's objectives – tuition fees, free care, carbon reduction – without huge borrowing. Nicola Sturgeon refused to apply 50% taxation to top earners, on the grounds that they'd simply leave the country and reduce overall tax revenues. She's been much criticised for that, but she's right to be cautious.

In one sense, the FM's tax policies are more radical than Labour's. She increased taxation on people earning less than £50,000, by freezing the threshold for higher-rate tax. Labour appear to be endorsing the Tories' tax giveaway by raising the threshold.

Jeremy Corbyn talks about the “billionaires” as if just shaking their pockets would finance the New Jerusalem. But there are only 151 of them in the whole UK and they would scarper before you could say inheritance tax.

Unlike the rest of us, capitalists are able to move at a moment's notice, and many already have. Like James Dyson of vacuum cleaner fame, and Britain's richest man, Jim Ratcliffe of Ineos. We may say good riddance, but the top 1% of earners currently contribute 28% of tax revenue. Some of that can be replaced by taxation on the rest of us, but it can't be done overnight. Labour plans to tax corporations and seize 10% of their assets, but many of them are only here by choice.

There was a reason why Labour handed out those pledge cards promising no tax increases back in 1997. People just didn't believe Labour could be trusted with the nation's finances. It took endless repetition of the tax promise to convince the voters that Labour had changed since the 1970s and were no longer the “tax and spend” party.

But now they are promising spending programmes which appear to be even greater than in the 1970s. Back then, Labour ended up appealing for a bail-out from the International Monetary Fund in 1976. That, plus the industrial chaos of the Winter of Discontent, led directly to Margaret Thatcher's election victory in 1979.

By 1997, Labour were electable again and under Tony Blair won their greatest ever landslide. That allowed them to introduced the minimum wage, impose a £5bn windfall tax on the utilities, and double NHS spending.

Taxes increased too, but largely by stealth, while the then Chancellor, Gordon Brown, uttered endless homilies about his love for “prudence” in fiscal affairs. Prudence has left Labour, and was last seen in Angela Merkel's Germany.

As in 1997, most of Labour's policies are pretty popular. Child care, elderly care, nationalisation, combating climate change. Labour is again moving with the grain of history in seeking to curb the privileges of capitalism and reduce the disfiguring social inequality.

Most voters support the NHS and want it funded properly. A 5% increase in taxation for those earning over £80,000 is also popular. But voters are intensely sceptical of the pledges politicians make at election time. The great danger for Labour is that people will just reject this as Corbynite pie in the sky.

Before the election, some Conservative insiders were saying that the reason Boris Johnson started talking about spending on 40 new hospitals, gigabit broadband, infrastructure and schools was to force Labour to promise even more spending to maintain their status as the party of the left. I don't know if that is entirely true, but it seems to have worked.

However, the Tories shouldn't be counting their chlorinated chickens quite yet. All the evidence is that the Tory polling lead is fragile, with a large number of don’t-knows. People don't much like Mr Johnson and the privileged class he represents. Mr Corbyn isn't trusted much either, but it would only take a few heartless remarks and insincere policies, for voters to decide: what the hell, at least with Labour I might get cheaper Netflix.

So, there is still time for Boris Johnson to throw away his poll lead, as Theresa May did, with a confused manifesto and a callous disregard for the less well-off. We are in a new age in which many of the Thatcherite verities are being discarded. People realise that we need to invest in infrastructure and in vital services like the National Health Service.

Austerity is dead. It is pushing up the daisies. But Labour doesn't want to join it by delivering what may soon be called the second longest suicide note in history.