IT took the SNP just a couple of minutes to pull James Kelly’s trousers down on Wednesday, metaphorically speaking. Scottish Labour’s finance spokesman was barely into his opening remarks in the party’s debate about public services when his indignity was exposed. This was not the plan.

Labour had billed the occasion as a high-profile ‘no confidence’ vote in the SNP’s draft budget for 2018-19.

Mr Kelly’s central line of attack, which he has been making since the Budget was published a month ago, was that it was a damp squib. It was mere “tinkering”, when the country was crying out for radical change.

In particular, he deplored the £700m “black hole” in funding for local government which would mean service cuts for the vulnerable.

All of which seems reasonable. In the past, the council umbrella group Cosla has said councils need £545m to stand still in 2018-19, but instead face revenue cuts of £153m. Cosla is expected to scale back its headline demand next week, but for now its £700m figure is the one Labour are running with. However what was also reasonable was for Labour to have a tax plan to fix things.

The SNP were girding themselves for one. On Monday, in the margins of a Holyrood committee, Finance Secretary Derek Mackay asked Mr Kelly if Labour’s plan was imminent. “All will be revealed,” replied Mr Kelly. Unfortunately all that was revealed was his nudity.

It wasn’t hard to discover, either. The SNP’s John Mason, a mild mannered accountant by trade, asked Mr Kelly the simplest of questions. “Could he spell out for us how he would raise that £700m?”

Mr Kelly’s response was that Labour would “take adequate time to put forward its proposals”. In other words, we can’t tell you yet because we don’t know.

Ignoring the laughter, Mr Kelly promised details later this month. “Consideration has to be given to such serious matters,” he said.

There is a certain logic to this, but politically it was downright inept. Mr Mason’s question had been in the post since the Budget came out and Labour started asking in indignant tones about that £700m.

The changes to income tax bands and rates proposed by the SNP should raise £164m. So who does Labour expect to tax and by how much to find the £700m on top of that to fill its black hole? That’s a vast difference, and not one it can possibly dance around after such strident demands for change.

One Labour insider said the ‘Nude Kelly’ episode reflected a wider “chaos” behind the scenes.

It seems that until a fortnight ago, Labour’s plan was not to have a tax plan. But now the plan is to have a plan, only there isn’t one. Not yet.

The Budget is a big test of Richard Leonard’s leadership. So far, he has painted his stall in primary colours, identifying big themes such as public ownership and redistribution, without offering much in the way of fine detail. The Budget process is where we see if he can do both.

Yesterday he announced a new Tax and Investment Commission to look at “a fair division of the nation’s wealth”. But that’s a medium-term project akin to the SNP’s Growth Commission. It won’t deliver the tax plan required in the next fortnight.

So what’s a leader under pressure to do? Labour’s recent manifestos aren’t much help. In the 2016 Holyrood election, the party backed raising the top rate of income tax from 45p to 50p on earnings over £150,000, with a penny on the 20p basic and 40p higher rates. But in last year’s snap election, UK Labour offered a wholly different model, with the basic and higher rates unchanged, a 45p rate at £80,000 and a 50p rate at £123,000.

When Nicola Sturgeon asked the opposition to put forward their ideas for “progressive” taxation in the autumn so they could be modelled by the civil service, Labour declined. Partly because it was leaderless, but also because of the muddle over two plans. Both also look decidedly less than radical next to the SNP’s two new bands in 2018-19, and higher tax bills on salaries over £33,000.

There is something Mr Leonard could use, though it may be painful. In November, as part of his failed leadership bid, Anas Sarwar devised a plan to raise £700m more in tax and sent it to Ms Sturgeon.

Rather than the five band model later proposed by the SNP, his was based on four - a starter rate of 15p up to £18,000, a 23p rate up to £42,385, a 43p rate up to £100,000 and a 50p rate above that. It would mean more tax for those earning over £28,000, Mr Sarwar said.

Perhaps ominously, Mr Leonard contributed nothing to the exercise.

However I hear Mr Sarwar’s tax model, partly because of a policy vacuum, is now gaining traction.

No doubt its adoption, or light revision, would be hailed by Mr Leonard as an example of comradely cooperation and magnanimity. No hard feelings here, he’d say. In truth, it would be an admission that in key respects Mr Leonard was less prepared for leadership than his rival.

The Sarwar plan was never modelled by the civil service, so it could also contain unintended consequences. But the alternative is trying to pass off dusty manifesto ideas as the new radicalism, or going into the Holyrood chamber as naked as Mr Kelly. I doubt anyone in Labour wants to see that.