WHO knew that Treasury Secretary Danny Alexander, seen nodding his head vigorously in support of George Osborne as he delivered the Budget, has a secret twin brother?

And, just to complicate matters, a twin brother answering to the name of Danny and belonging to the Liberal Democrats.

That can be the only explanation for why Alexander, who chatted animatedly with David Cameron throughout Osborne’s speech, swanned into the Commons yesterday to deliver his own fantasy Budget.

How did he swing this? Predator Jimmy Savile is dead, so he can’t have been a lucky Jim’ll Fix It contestant.

Perhaps TV game show Stars in Their Eyes has been revived and his plaintive pitch of “Tonight, Matthew (or Speaker John Bercow), I shall be George Osborne” was accepted to provide some light entertainment for jaded MPs.

That is surely more likely than the alternative that, as an endangered species likely to lose his seat to the Scottish nationalists in May, he should be comforted by having an each-way bet, backing agreed coalition government policy one day and then offering his own wish list the next day.

Whatever, this episode was unique in parliamentary history.

No other minister has ever been permitted to present an alternative Budget after having been part of the team that drew up the real thing, posed for media snaps with the Chancellor and his red box and then sat on the front bench alongside him to show a united front.

Bercow’s warning that Alexander’s statement should be ministerial and should refrain from making party political points clearly fell on deaf ears.

Making a virtue of his party’s tendency to position itself between the two main parties, he pledged “a plan that borrows less than Labour and cuts less than the Conservatives.”

Warming to his theme, he spurned Labour’s assessment that planned Tory spending cuts would take Britain back to the 1930s while also rejecting Osborne’s suggestion that they would equate to a return to the days of Gordon Brown.

So what to do? Split the difference — how about 1964? That should be about right.

By coincidence that was the era of the BBC TV homelessness drama Cathy Come Home that spurred the establishment of housing charity Shelter, offering Alexander the chance to stake his claim to human decency.

“The era of Cathy Come Home is not my vision for the future of Britain,” he pontificated.

Possibly not the future but certainly the present, since homelessness and overcrowding grow ever more serious by the day.

Council housing is sold off, hardly any new local authority homes are built, tenants are evicted or impoverished by the bedroom tax and Alexander’s government primes house price inflation to the delight of speculators and the despair of low-paid workers.

He has the temerity to promise a “mansion tax” on high-price properties, but Liberal Democrats voted as a bloc two years ago against a Labour motion proposing its introduction.

They branded it “infantile,” accusing Labour of “trying to drive a wedge” between the coalition partners and reiterating their support in principle for a mansion tax.

The Liberal Democrats’ Alice Through the Looking Glass stance on the mansion tax mirrors Alexander’s “Look at me. I’m a Chancellor” make believe.

Latter-day efforts to distance themselves from the Bullingdon boys cannot hide the reality that their consistent priority throughout this Parliament has been to assist the Tories to carry out their anti-working-class austerity agenda.