ALWAYS beware politicians bearing round numbers on the public finances. A £100m boost here, or a £500m fund there, invariably turns out to to be a mix of rehashed old money and wishful thinking.

So when Scottish Labour this week unveiled an alternative budget promising a “near £1bn stimulus” you knew at a glance it was a dud. Deeper reading proved it.

This year, for the first time, there’s a newcomer to the Budget process - a referee. The independent Scottish Fiscal Commission (SFC), placed on a statutory footing at Labour’s insistence, vets the Government’s tax plans and assesses how much they will raise in the real world, not just on paper, and that number is all that ministers are allowed to spend.

Any heroic optimism about tax revenue is brought to down earth with a bump. When the draft Budget came out in December, the SFC said the SNP’s income tax changes would raise £164m in 2018-19. In theory, on what is called a “static model”, they raise £215m. But as the rich will try to minimise their bills, around a quarter is lost to “behavioural effects” ie tax avoidance.

That lost £51m was based on the SNP adding just 1p to the higher 40p and additional 45p bands. The SFC believes hefty rises mean more aggressive avoidance, and possibly less money that no tax rise at all.

Labour’s alternative budget boasted it could raise £540m. But it was based on a static model. It took no account of human behaviour, despite the idea being to soak the rich with a 45p rate starting at £60,000 (instead of £150,000) and a 50p rate above £100,000.

If Labour were in office, the SFC would have told them their figure was a fantasy. And it would be the SFC’s hard-nosed number - possibly a negative one - that went in the Budget, not Labour’s gimmick.

There were other problems. Around £150m of extra cash was meant to come from new council-set tourism and land value taxes. But both would require primary legislation, so would never be in place for the new financial year. The SNP reckoned Labour’s budget would raise less than £300m rather than the £1bn claimed.

Unsurprisingly, the SNP passed on Richard Leonard’s “radical” offering and struck a practical deal with the Scottish Greens instead.

But Labour, like the semi-engaged LibDems, do get credit for trying. They put a plan down on paper and placed it in the public domain, albeit flawed. Some of the spending ideas also have cross-party sympathy, such as an extra £5 a week on child benefit, higher public sector pay and more cash for councils. You could imagine some of the new tax ideas being adopted in the medium-term.

The worst offenders in the Budget process by a mile were the Scottish Conservatives. Holyrood’s official opposition effectively quit the field. After urging more spending yet lower taxes, they offered little more than vapid slogans about efficiency and improving economic growth

As the SNP’s Kenneth Gibson said in Wednesday’s debate, “The Tories make Labour’s proposals seem rational and coherent… the Tories have failed to engage.”

When SNP MSPs pressed the Tories on how they’d cope with the £550m drop in public spending implied by their hazy vision, the reply was always the same. “It is dead simple,” as Murdo Fraser put it. “We would cut out the waste and the vanity projects and we would grow the Scottish economy.”

A little more detail comes from a December press release identifying £210m of “efficiency savings that the Scottish Government could be used to invest in public services”.

This included saving £88m by not using any agency staff in the NHS, though presumably someone would have to do their work instead.

Saving £38m by ending the vexed problem of delayed discharge in hospitals. A doddle, obviously.

And saving £13m by having Scottish Enterprise invest only in firms that succeed and never fail.

While some other ideas might have yielded a fraction of the sums claimed, most were either old fashioned cuts or Utopian tripe relying on error-free bureaucracy, as deluded as Labour’s idea of the rich racing to hand over more tax.

On the economy, the big Tory plan is basically attacking SNP tax rises, as these are assuredly bad for it. Other bright ideas are TBC.

This is woefully thin stuff. As Finance Committee convener Bruce Crawford reminded MSPs, as Holyrood’s fiscal powers increase, the Budget gets ever more complex,

Saying it’s “dead simple” to effect change is insulting the intelligence.

There is a reason the Greens keep striking Budget deals with the SNP. Clearly there’s common cause on independence and “progressive” tax and spending, and a related desire to deny other parties a win.

But the Greens also do their homework. They produce detailed policy papers and they engage with the Government. They put a shift in and, lo, they get results.

Meanwhile, this year Labour pretended to engage, and the Tories pretended to be an opposition.

Ruth Davidson insisted again this week that she’s laser-focused on becoming First Minister in 2021.

In which case it might be an idea for her party to start acting like a thoughtful government-in-waiting, not sour and shallow bystanders.