Darren Jones has spent much of the past few months doing the traditional, hard-nosed job of a Treasury chief secretary – fighting line-by-line budget battles with ministers. But with last month’s fraught spending review over, he has turned his attention to an issue closer to his heart for his latest policy.

“The council estate I grew up on was one of the neighbourhoods that was picked by the New Labour government because it was so deprived, essentially, in terms of income and educational outcomes,” he says.

The 38-year-old MP for Bristol North West grew up in a flat on the Lawrence Weston estate. His mother was a hospital administrator and his father a security guard, and he has previously spoken about how money was sometimes tight at home. In his maiden speech in the House of Commons, he proudly mentioned being the first Darren to be an MP.

Jones cites Labour support for local community group Ambition Lawrence Weston – still thriving today – as well as a taxpayer-funded “gifted and talented” scheme, as crucial in helping to signpost him towards university, as the first in his family to go. He went on to qualify as a solicitor, and was an in-house lawyer for BT before being elected in 2017.

With projects like these in mind, he is now announcing that the Treasury will invest £500m over a decade, alongside private backers, in a new “social outcome partnership” to fund grassroots projects tackling child poverty.

He says he made it a personal mission since arriving in the Treasury last summer to dramatically expand this form of “social impact investment”.

Pioneered by Gordon Brown’s Treasury, it is an approach that involves private investors matching taxpayer funding for neighbourhood-level anti-poverty projects.

These backers earn a modest return, but only if the scheme meets specific targets – which might be, Jones says, getting a certain number of children into college, or university, or parents into secure jobs, for example. The fund is expected to be the largest such vehicle in the world. Jones hopes it will be worth £1bn in total.

“It’s really trying to just unlock those opportunities, like it did for me,” he said. “I’ve now had a great career and I get to do this job. And a lot of that stems from what the New Labour government did. So essentially this type of funding mechanism, this investment into tackling the root causes of poverty is something that’s very personal to me.”

The detailed proposal emerged from a social impact investment advisory group, set up by Jones last year. It was due to wind down this summer, but will now continue at his request, to draw up plans for a more general template for social impact projects, that could be applicable across Whitehall.

The approach is of a piece with Jones’s relentless optimism that, despite governing in a time of straitened fiscal circumstances, this Labour government can find innovative ways to do more with less.

“The public really want things to change and get better and we agree with that, but because we’ve inherited these really challenging economic circumstances, we can only do so much with the traditional mechanisms. So we’re open to trying to find other ways of getting stuff done for the country and improving people’s lives,” he says.

That includes considering launching a new generation of public private partnerships, which the government hopes could be used to fund the new neighbourhood health centres, that are at the heart of Wes Streeting’s 10-year plan for the NHS.

Jones insists these will be narrower and less complex than the controversial projects launched through the private finance initiative (PFI) under New Labour, some of which have saddled operators with hefty bills.

“We are not doing PFI. We’re not doing that, these very complicated contracts. We’re not talking about hospitals or schools or prisons or anything like that.” But he added: “If there’s an innovative way of delivering a key objective for us, that’s what we’re trying to make happen.”

For the same reason, he is a keen advocate of the wider use of AI in government, though he insists it must be “an enabling tool, not a replacement” for civil servants.

While he is upbeat by temperament, Jones acknowledges the fearsome squeeze the government faces, in the week the Office for Budget Responsibility warned that the UK’s public finances are on an unsustainable course, and after his boss Rachel Reeves’s tearful outing in parliament sent bond markets reeling.

“Debt is nearly 100% of GDP: it’s expensive, it costs a lot of money. We’ve had poor productivity growth and wage growth in the economy for the last 10 years. And so what the government, what Rachel’s trying to do is walk that tightrope out of that kind of fiscal headache,” he says.

The ambitious Jones was touted by bookmaker Coral earlier this month as the second most likely successor to Reeves, after the Cabinet Office minister, Pat McFadden. But he is fiercely loyal to his boss, repeatedly underscoring the importance of her fiscal framework.

“Fiscal rules are not self-constraining tools to just flagellate yourself with. They’re there for a reason,” he says.

He refuses to be drawn on the prospect of tax rises in Reeves’s autumn budget, which are widely expected, in the light of the U-turns over welfare cuts and the winter fuel allowance, and continued weakness in the economy.

But he does repeat the chancellor’s promise that after the welfare cuts, “we’ll make sure that’s fully funded in a proper way at the budget” – but without jeopardising Labour’s manifesto promises not to touch the major revenue raisers of income tax, employee national insurance or VAT. “We’re going to honour our promises.”

Challenged as to when voters will start to feel the benefits of Labour’s investments in infrastructure, housing and industry, Jones says the government will make no apology for taking a long-term view.

But he also points to more immediate plans, set out in the spending review, to repair public buildings. “It’s not very sexy, but we put loads of extra money into maintenance,” he said.

He then goes on an enthusiastic digression about how Labour’s long-term funding for pothole maintenance, in contrast to annual budgets under the Tories, will allow local authorities to “actually dig out the bit of the road that’s broken and relay it properly,” rather than just, “get the guys to come out and put tarmac in the hole”.

Posing before the vast oil painting of Queen Victoria that dominates his airy Treasury office overlooking St James’s Park, Jones gives a final demonstration of his caution with public money. The imposing work was chosen from the government art collection by Jones’s predecessor, Laura Trott. He is not a big fan, he says. “I thought it would cost a fortune to get it out, which is the reason I left it.”