Rory Stewart has just been appointed international development secretary. Last week, he explained to Katy Balls why he would make a good Prime Minister:

Almost nobody in Westminster admits to wanting to be prime minister. Rory Stewart is a cheerful exception. Most leadership hopefuls prefer to plot in dark corners and woo supporters in candlelit bars. The Prisons Minister is happy to sit in the sun in Hyde Park and talk openly about his ambition.

It’s a tricky time for this country, he says. ‘In a normal situation I probably wouldn’t want to run.’ One of his friends thinks he’s mad: what’s the matter with just being MP for Penrith and the Borders? Why seek No. 10 now? Isn’t it a completely thankless task? ‘But I’m the opposite way around,’ he says, as we sit by the Serpentine lido. A voice for the pragmatic middle ground will be needed when Theresa May departs Downing Street. He’s offering his.

It’s a job Stewart believes he could be good at. ‘There are some things I would be really, really bad at. You would not want to hire me to run a ballet company, I’d be really rubbish at that. I don’t like ballet. I am bored by it. I’m almost tone deaf. I feel I want to die when I watch the ballet. It would be a really, really bad mistake to hire me to run a ballet company. But if you want someone who really enjoys doing stuff and loves government and is really proud of the country and feels that’s their thing, I’m really enthusiastic.’

After working as a diplomat in the Middle East, Stewart entered parliament in 2010. Now aged 46, he feels ready for the next challenge. ‘Difficult periods need different types of people. One of the reasons why I would be tempted towards this job is that we desperately need to rebuild ourselves internationally after Brexit. I am one of the only people in Parliament who is a genuine specialist.’ He was a regional governor in Iraq as it descended into sectarian violence: not the most encouraging analogy. But Stewart says that experience means he is good in tight spots. ‘That’s been my life. I’ve written four books about it, done three television documentaries about it, I’ve taught it at Harvard, I’ve spent more than a decade living in funny countries and working there. I’ve done it as a developer, as a diplomat, I’ve done it in war zones.’

Stewart’s CV reads like that of a John Buchan character. The son of an MI6 intelligence officer, Stewart spent much of his childhood abroad. He went to Eton a few years below Jacob Rees-Mogg, then enjoyed a gap year stint in the Black Watch. He has advised the Obama administration and was, for a spell, a tutor to princes William and Harry. He has trekked the 6,000 miles from Turkey to Bangladesh. That was tough, though not nearly as difficult as his latest challenge: trying to defend the Prime Minister’s beleaguered Brexit deal in public.

He believes that May’s deal is a much-needed compromise. That hasn’t won him many fans. ‘It is a really good deal,’ he insists. ‘I just think although people pretend they want a Brexit deal it turns out that far too many Remainers simply do not want to accept the result of the referendum and far too many Brexiteers have convinced themselves that no deal is the sensible, practicable thing to do and I don’t think it is.’ His mother Sally, who voted Leave, is among those who remain sceptical though Stewart is doing his bit to win her around.

‘She spends a lot of time on Brexit websites that tell her that the deal is rubbish. I then get the deal out and I go through it and she says, “Well why does nobody ever say that, darling?” So I say, “Well I’m saying it”.’ He thinks May ought to stay on until the first phase of Brexit is complete — even if that means relying on Labour votes to pass a deal. Part of the reason Stewart is interested in the role of prime minister is his worry about the direction the party could go once she departs. ‘The biggest challenge in the world at the moment is extreme polarisation of politics. It’s the one thing that Britain has had a genius in avoiding since 1688 so resisting the extremes is not just saving the country, it’s saving the constitution, it’s saving the politics, it’s saving what makes a nation and a community.’

He fears that were somebody ‘like Dom [Raab] or Boris’ to go for a leadership challenge now they would campaign for what he regards as the extreme of a no-deal Brexit. Such a move, he argues, would defer not just Remain voters but young people in general. ‘Politicians want to believe “I can be a hard Brexiteer and I can also be modernising, progressive” but it will be a very, very difficult trick to pull off.’

He is sceptical of the rhetoric used by some Brexiteers — such as Raab when the pair debated one another at a Spectator event before Christmas. ‘His basic response seemed to be “Don’t talk down Britain, believe in Britain”. Politics is very exciting but in the end Mrs Thatcher said the most sensible way to think about it was in terms of a household and you need to keep translating anything a politician says into what would make sense in a household. So, if you said to me you are never going to be able to get those three black bin liners into our rubbish bin and I said to you, “Believe in Britain, don’t talk down the rubbish bin”, I’d either be making a joke or I’d deserve a kick up the arse.’

While Tory voters might be seen as a group partial to messaging on bins, this is a pitch that is unlikely to sit well with the eurosceptic membership. Stewart knows this and admits whether or not he has a chance at the top job depends a lot on timing. ‘If the only thing that people care about is delivering a no-deal Brexit and not a Brexit deal then someone like me doesn’t stand a chance. If on the other hand you have got Brexit done, or you have got the first stage of Brexit done, then I think somebody who appeared exciting and practical would be quite appealing and I think at that stage people might be a bit weary of Brexit, they might be quite keen to talk about something else.’

A Labour supporter as a teen, Stewart’s politics do not come from economics textbooks or Ayn Rand novels. ‘There are types of right-wing Conservatives who are very unsentimental and think the whole world comes down to economics and money. I’m not one of those.’ His ‘two big things’ are ‘pride and decency’ and he applies this to his current brief in the Ministry of Justice, where he has promised to improve prison standards or resign later this year.

Nor is he a radical. On domestic politics, Stewart is against drugs legalisation, wants two million more homes built but not on the green belt and describes himself as ‘sceptical’ on HS2 even though his constituency is meant to be a beneficiary. If he were PM for the day, he says he’d be tempted to gather every civil servant in London in Hyde Park and lay out a huge sign right the way across the park saying ‘Britain wasn’t built in a day’.

Nor, he accepts, is a leadership campaign. ‘If I think about me, I have clear disadvantages. I am an Old Etonian, I voted Remain, I have not been in Cabinet. But there are other bits of me which I don’t actually talk about but which are quite different. I don’t talk about setting up a charity in Afghanistan. I don’t talk about what I did in Iraq. I don’t talk about what I did in Indonesia. I don’t talk about my books. I have tended to try to concentrate on learning about being an MP.’

He says that his experience outside politics ‘does give me a broader experience of actually running things outside Parliament, a sort of hinterland. I think I’m not stupid and I’m not bad at getting things done’. Deliver a Brexit deal, he says, and lots more can be done. ‘There is no reason why Jeremy Corbyn can quadruple the number of members of the Labour party and we couldn’t quadruple the membership of the Tory party.’

Stewart spent his Easter holidays in Cumbria. He posted a video on social media of a lamb gambolling in a barn. He also planted trees, which set him thinking. Trees, he says, ‘are perpetually astonishing’ because they grow in all sorts of ways over decades.

‘The big question for Britain is not really what could I do if I was prime minister in five, ten years — but what does the country look like in a hundred years? If I were lucky enough to be prime minister, I’d want to do a lot of things you wouldn’t see the results of while I was there.’ Heathrow runways, 5G networks, clean air all take time, he says.