TAM Dalyell was wont to deploy a charmingly eclectic blend of interrogatory techniques in the Commons.

Sometimes, as with his own West Lothian Question, he would opt for attrition, pursuing Ministers endlessly.

However, I preferred his approach succinct. Responding to an initial written question, a Minister would read out a turgid civil service response, then subside gratefully, hoping for a lengthy supplementary to catch his breath.

Up from the Labour side would bound Tam, to declare in his sonorous Etonian tones: “How much?” Or, even better: “Why?” Cue startled Minister.

I thought of Tam when I pondered the question of a putative co-operation deal between the SNP Scottish Government and the Greens. Recalling my journalistic coaching, I asked: “Who, what, when, where, why?”

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We know who: two parties, of different stature, supporting independence with varying degrees of fervour.

We know what: a co-operation agreement short of full coalition. Eventually, there might be Green Ministers. For now, too soon.

It would be a little rough on newly appointed SNP Ministers. “I thank you for your loyal fortnight of service. Would you please now usher in your Green replacement, closing the door behind you with a minimum of noise pollution?”

More to the point, the Greens stress they would reserve the right to dissent on certain specified issues. Which means no collective governmental responsibility. Which means no Ministers, for now.

When? The talks are said to be at an early stage with months of discussion to come. Where? Holyrood.

The big question, though, is why. One observer tells me that the Green co-leaders, Patrick Harvie and Lorna Slater, seemed faintly taken aback at the initial “talks about talks”. Warily and sensibly, they stressed the need to consult colleagues.

However, they have agreed to continue negotiating, noting en passant that they have secured concessions in recent Budget deals.

There is history here. In 2007, Alex Salmond led a minority administration. He tried to strike a partnership deal with the Liberal Democrats. That fell apart: key LibDems found the SNP and independence fundamentally unpalatable.

Mr Salmond then turned to the Green MSPs. All two of them. The leader Robin Harper, with his permanent smile and Dr Who scarf. And Patrick Harvie, an astute politician even then.

Again, a full partnership proved impossible, owing to a dispute over transport policy among other issues. A compact was signed to support the election of Mr Salmond as First Minister.

That was not really needed: subsequent history proved that the party with the most MSPs would, in practice, be permitted by Parliament to enter office.

The compact was relatively insignificant, politically and numerically. It was hoped it would, as a minimum, guarantee support for John Swinney’s annual Budget. But, in 2009, the Greens refused, despite last-minute negotiations, to back the finance package.

They said they had been sold short. Mr Swinney was, to put it mildly, less than pleased. That memory no doubt lingers in the mind of the Deputy First Minister as he contemplates the new round of talks.

So, again, why? Especially now, when the SNP are just one short of an overall majority in Holyrood.

I sought answers from my Herald Podcast panel; among the number, Kezia Dugdale, who formerly led Labour and now heads the John Smith Centre. She disclosed that she had conducted extensive research, including a phone call to a pal in New Zealand, where there has been cross-party co-operation.

She was advised that the Greens there felt they gained clout and influence. Labour leaders felt they gained extra security, including a pact which obviated the need for tense annual Budget talks.

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The deal has persisted, even though Labour won a majority last year. Some Greens are discontented.

Scottish Green sources point to the New Zealand model, stressing the capacity to criticise the administration. They are less enthusiastic about the example of Ireland where some Greens fear their ideology risks being swamped in a compact with centre-right parties.

Either way, it is, partly, about money. Nicola Sturgeon and Kate Forbes want to pre-empt the annual agony of Budgetary negotiations by securing potential support in advance.

Not an easy sell. SNP Ministers are adamant they will stick to an income tax freeze for the duration of this Parliament. Greens favour a more radical approach to revenue, as they would see it, including a possible wealth tax if a way could be found.

But compromise could be reached. There will be other obstacles, such as Green opposition to road building. The FM said this week that dualling the A9, for example, will not be halted.

Then there is the North Sea. The parties differ over the pace of shifting away from oil and gas. With the COP 26 UN climate conference due in Glasgow, the SNP may be ready to give yet more ground, while protecting north-east jobs.

Always, of course, there is independence. Would a compact add more impetus to the case for indyref2? Not, I suspect, in terms of influencing the UK Government. But it would perhaps be a reminder of the diversity of support for independence.

There is, however, a third motivation, beyond cash and indyref2. The SNP have been in power for 14 years. The previous Holyrood session had a toxic tang: Brexit; independence arguments; and Nicola Sturgeon gazing fretfully over her shoulder at her ever-present predecessor.

I believe the FM wants a fresh approach. Literally, a new politics. Sceptics will say this is purely about presentation. I think it is also strategic. By making a choice, by making a new alliance, Ms Sturgeon hopes to banish staleness from her own party, and simultaneously challenge rivals to refresh.

She hopes, in so doing, to isolate the Tories. They respond by warning that the Greens will push her in an anti-business direction.

She hopes, also, to challenge Labour, over policy and indyref2. Anas Sarwar was, understandably, ill-prepared. His first response was simply to revive an old Labour theme, deriding the recreation of a spending cuts coalition.

He will need a new tone, new phrases. However, if a deal emerges, he and others will still rightly demand: “Why?” And: “How much?” Tam Dalyell would be proud.

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