So often a beige-grey plod, First Minister's Questions was a kaleidoscopic romp today.

First it was so-so, then it was dire (or so it appeared), and then, against all expectation, it became something intriguing and substantial.

The so-so bit was Labour's Johann Lamont. Last week she tried and failed to have health secretary Alex Neil sacked by Alex Salmond.

This week she tried and failed to have Alex Salmond sack health secretary Alex Neil. She's unpredictable, you have to give her that.

A raft of new statistics showed the NHS was "going backwards on the FM's watch", she said.

But Mr Salmond knows all about going backwards. Instantly, he cited waiting time figures to her from 2006, when Labour was in power.

Eight years ago, Labour were delighted when the service was poorer than it is now, he observed.

"I'll be delighted when he stops displaying disgraceful complacency," Ms Lamont snapped.

The pair then played a quick match of boffin tennis, trading quotes from top docs until everyone else felt anaesthetised and the FM felt able to sneak out an admission of imperfection.

The statistics were "not ideal" he conceded, but they were all better than under Labour.

Ms Lamont, as ever, was unimpressed. "He's more at home in the imagined would after a Yes vote than the real world," she seethed. "Will he get on with the day job?

At which, Mr Salmond did get on with his day job - tearing apart Ruth Davidson. The Tory leader's question was on the cost of setting up an independent Scotland.

Given the Treasury had just been nicked for making up the estimates, this could not have pleased the First Minister more.

He seemed barely able to contain himself as Mr Davidson asked for a breakdown of the £250m "reasonable estimate" he mentioned yesterday.

Did she not know the figure came from the LSE's Professor Patrick Dunleavy in response to the Treasury "badly misrepresenting" his work by using it to claim the cost was £2.7bn?

There was a "basic unwisdom" in introducing Prof Dunleavy" to FMQs, he chortled ruddily.

Ms Davidson's comeback was that Prof Dunleavy's figure, in his own words, was a "guesstimate".

The SNP benches, front and back, started hooting with ironic laughter.

"Order! Let us hear Ms Davidson," cawed Presiding Officer Tricia Marwick.

"I find it worrying, that the First Minister has no intention of telling the people of Scotland how his paperclip economics add up," the Tory leader carried on to more derision.

But just as it seemed her high-wire act was going to end with a mop and a bucket, she produced John Swinney's infamous leaked memo of 2012 on the true costs of independence.

She quoted the Finance Secretary: "Work is currently under way in finance and the Office of the Chief Economic Adviser to build a comprehensive overview of the institutions, costs and staff numbers which I will draw together and provide an update to Cabinet on in June."

That was June 2012, she reminded the FM. "The cabinet has the numbers. The First Minister has the numbers. Why will he not let Scotland see the numbers?"

The First Minister's response was instructive. He wouldn't go anywhere near the subject.

Instead there was a cheeky theatrical monologue on the Treasury's misfortunes.

"It was UK government who introduced Prof Dunleavy, who I must say is an estimable person, I've never met him, but he sounds like the sort of person I want to meet!" he roared.

And then there was the other Prof, Professor Young, who the Treasury also cited and who said: "The UK position is to make pessimistic predictions, warnings and the occasional threat."

Shaking at his own mirth, the FM added: "Prof Young is somebody I want to meet as well!" Fixing his opponent, he sneered: "I think it's time to revise her questioning strategy."

SNP MSPs cheered like a Cup Final crowd. When LibDem Willie Rennie started on the same subject, the Nat stands roared even louder.

"Let us hear Mr Rennie, please," said the PO. "More! More!" came SNP cries.

Ms Marwick whacked her gavel. "Come on! Behave!"

It was irrelevant, for Mr Rennie was steely. Closing the jaws of a Tory-LibDem trap, he pointed out the transition paper raised by Ms Davidson was written two years ago.

"Will the First Minister tell us where it is?" The simple questions are always the best.

"Well, eh, eh, I would say, ah," stumbled Mr Salmond, before again dodging the point.

He talked about his own economic analysis, he talked about the Treasury's, he talked about a more prosperous Scotland under independence.

He conspicuously did not talk about the transition costs commissioned by Mr Swinney.

There was a subtle change in the chamber. An almost imperceptible shift in mood that only the startled political animals scented.

Suddenly Mr Salmond was on the backfoot. Mr Rennie pressed his advantage.

"The answer to 'Where is it?' was very long but did not answer the question...

"The First Minister scoffs, but he needs to answer the question. If there is work, why will he not show it? If there is not, why has it not been done?"

Will he agree to publish the figures? The SNP backbenches fell supernaturally silent.

A rattled Mr Salmond sped through the highlights of this week's SNP economic analysis on life post-Yes.. £1000 for every man, woman and child.. a fairer society.. a better future for the people of Scotland.. er, what was the question?

It was a brave fist but, by the end, it appeared Mr Salmond did indeed have his own estimates for the start-up costs of independence, but instead of publishing them was sheltering awkwardly behind Prof Dunleavy's £250m guesstimate.

Even with the success of his 5:2 diet, it was too small a fig leaf to hide his embarrassment.