IT'S always nice when you find that fiver in the back of your jeans, or those pound coins behind the sofa, along with biros and furry sweets. But imagine finding £60bn down the back of the cushions? That's the cumulative amount that the Office of Budget Responsibility has apparently overestimated UK government borrowing over the past three years.

This means that the annual deficit – the prime target of fiscal policy for the past decade – is around half what the government thought it was. The UK government is only borrowing around £25bn this year instead of going on £40bn. It seems that the real culprit was the Office of National Statistics which has been under-estimating the size of the UK economy and the tax revenues that it generates. In many businesses, an accounting error of such magnitude might be a sacking offence.

But this is government, where no one is to blame. And anyway the Chancellor, Philip Hammond, is cock-a-hoop as they used to say in football commentaries, at his windfall. It has allowed him to declare the end of austerity, cut taxes (for viewers in England only) by bringing forward the raising of the higher rate tax threshold, throw a couple of billion at the broken-backed Universal Credit welfare reforms (“it's here to stay”), and promise £20bn for the National Health Service.

It was the ultimate get out of free card for “Fiscal Phil” as he styled himself yesterday in a Budget that was long on gags if rather shorter on reality. Austerity has not of course ended, even if the deficit is set to rise marginally next year, for the first time in a decade. The freeze on working age benefits, like job seekers allowance, has another year to run, and local councils are still grinding through spending constraints that has led some of them to go bust, in England at least.

But relief is at hand. The Chancellor made a groan-worthy bid for tabloid headlines by announcing business rates relief on public conveniences. Britain may be about to go down the toilet after Brexit, but at least we'll be able to spend a penny without that adding to the cuts in social care.

“It's the only thing that hasn't leaked,” he quipped, in what sounded like a dig at the Prime Minister. Theresa May had pre-announced many of the headline items in the Chancellor's Budget, like the freeze on fuel duty and the extra for the NHS. Which, as a number of analysts pointed out, isn't all that earth-shattering at 3.4% per annum. The increases in NHS spending under Labour were far greater than that.

But of course, according to the Chancellor, austerity was all the fault of Gordon Brown throwing our money down the pan in his Venezuelan years. You might have thought that the 2008 financial crisis was caused by excessive bank-lending, sub-prime mortgages and the behaviour of the likes of Fred Goodwin of RBS. But no, it was “Labour's Great Recession”, according to Mr Hammond yesterday.

Jeremy Corbyn was furious about that in his Budget response in the Commons, which as usual failed to soar to oratorical heights even if the indignation meter was off the scale. The Leader of the Opposition was also furious about the Chancellor's claim that “income inequality is less today than under any of the last Labour governments”.

This is actually true – in a way. The bottom 20% of earners have indeed seen their earnings increase faster than the top 20%. But this is largely because median wages have been frozen for the last decade, while there have been significant increases in the Living Wage – up to £8.21 after yesterday's hike. So, it is more statistical than real, and is actually a measure of how poorly the economy has performed for middle Britain – the very “hard-working families” the Chancellor appealed to yesterday.

Indeed, you could almost say that the relative increase in the earnings of the bottom 20% has largely been paid for by the longest pay pause in 200 years. It doesn't alter the fact that the top 1000 richest families in the UK have doubled their wealth since the crash, or that CEOs now earn 145 times the average pay of their employees. The inconvenient truth is that people on higher incomes have done extremely well since the crash and will continue to benefit disproportionately from the Chancellor's windfall.

It will also help Mr Hammond's Scottish equivalent, Derek Mackay, out of his own budgetary hole. That £20bn for the NHS should push around £2bn north through the magic of Barnett Consequentials. It couldn't come at a better time. Following Audit Scotland's report last week saying that the Scottish NHS was “unsustainable”, Nicola Sturgeon's hard-won reputation for sound management of the health service was beginning to look distinctly shoogly. Wisely, Mr Mackay has promised to put every penny of the UK NHS uplift into health, where it belongs. It's not entirely clear that the Scottish government has always done this in the past, even if it has kept health spending ahead of inflation.

The Chancellor said that Scotland stood to gain nearly £1bn in extra cash next year alone, when other factors were included. The slashing of business rates by one third for small companies will also have lucrative consequentials, though Mr Mackay isn't quite so certain that this will all be handed on. To be fair, the Scottish government has led the way in helping small businesses weather the storm on the high streets. Its long-running small business bonus kept many village shops from going under, and it seems that the UK government has noticed.

There will be no changes to tax reliefs on North Sea Oil industry which will also come as a relief to the Scottish Finance Secretary. And there is no increase in whisky duty, to allow us to have “a wee dram for Ruth Davidson” as he Chancellor put it. What he didn't say is that we may need a drop of the hard stuff to deal with Brexit.

This was the oddest Budget I think I've witnessed. This is partly because of the unreality of the underlying fiscal figures: I mean, if the OBR can get its numbers so massively wrong this year, isn't it possible that, in future, it may make a similar mistake the other way?

But it is also the last Budget before everything changes on 29th March next year as Britain crashes out of the European Union. The Chancellor hardly mentioned Brexit in his speech, except to offer the alliterative forecast of a “double deal dividend”. He gave no indication of how this might materialise, or how it squared with his own Treasury's forecasts of the negative impact on the UK economy of leaving the single market.

Philip Hammond has made clear that his Budget only works if there is a favourable Chequers deal, guaranteeing friction-free trade with the EU. That looks distinctly unlikely. On the very day of the Budget speech, the former Foreign Secretary, Boris Johnson, penned another epistle in the Daily Telegraph calling on his Tory colleagues to “chuck Chequers” which he believes will leave Britain at the mercy of malign Brussels bureaucrats, determined to bring Britain down.

We may look back on this Budget as the last of its kind before Britain enters a new era of extreme economic turbulence and self-inflicted uncertainty. Or as the Chancellor might have put it: “Apres Moi, le deluge”.