Budget 2016: How do the sums stack up?

My favourite quip on the Budget from the various think tanks and financial institutions weighing in on Mr Osborne's eighth go at shaping our financial destiny comes from Torsten Bell - director of the Resolution Foundation (and a former aide to Ed Miliband) -"The sofa can go from being a cashpoint to a pickpocket overnight."

He is referring, if you keep up with the minutiae of these events, to the Office for Budget Responsibility (OBR), the independent budget watchdog, which today reversed its unexpectedly optimistic assessment of the economy made in November, suddenly taking away twice the £27 billion which it had previously found 'down the back of the sofa' leaving Mr Osborne with a massive hole in the country's finances to fill.

The OBR reckons that the economy will grow significantly less quickly than it thought in November, it highlights a dip in productivity, compounding the problem long-term as the dampened economic activity feeds through into lower tax receipts for the government. At the same time, Mr Osborne is giving away some very expensive policies, like increasing the tax-free allowance.

So how is he going to pay for all this, to put the country's books back on track for a surplus in 2019? It will involve an astonishing swing from a 21 billion deficit the year before and is making many economists raise a sceptical eyebrow.

For a start, he will extract another £3.5 billion in savings from those poor 'unprotected' government departments like Justice, the Department for Business Innovation and Skills, and the Department for Work and Pensions - although he puts off this extra burden of austerity until the very last year of the parliament. This is odd, adding pain just before an election, and suggests Mr Osborne is hoping the sofa may come up trumps again.

The Chancellor will also raise cash from a stiffer corporation tax as well as some nifty shifting of revenues he was due to book earlier in this parliament - further helping to achieve the turnaround to surplus.

I'm exhausted just trying to unpick these efforts to make the sums stack up. Heaven knows how the officials feel who dreamed it all up. I suspect one might find them napping on a sofa somewhere.