Fish and chip shops are dangerous places for politicians, as Labour spin doctors know. Peter Mandelson’s image never recovered after he mistook the mushy peas for guacamole at his local chippie in Hartlepool; and in these health-conscious times, it wouldn’t do for an ambitious young cabinet minister to be seen tucking in to such fatty fare with too much alacrity.

So it’s a surprise to find David Miliband, the environment secretary, carefree when he arrives for the reopening of Colmans of South Shields, an award-winning fish and chip shop in his constituency.

Indeed Miliband is practically skipping with enthusiasm as staff prepare the red ribbon, and enthusiastically accepts a giant platter. But then this is a chippie with a difference. As proud notices on each table declare, here they use only wild fish from sustainable fishing grounds; the food is fried in additive-free vegetable oil that’s low in trans fats; and leftover oil from the deep fat fryers is collected and made into biofuel. Beat that.

“The food here’s fantastic, and I’m having full lashings,” says Miliband munching on a chip. “You’ve got to try the fish. It’s so good.”

It must be tiresome having to be a model of environmental virtue even when you go for a fish supper. Few of Miliband’s cabinet colleagues are under as much pressure to practise what they preach every day. But Miliband — tipped as a possible future prime minister — is hounded for every environmental slip-up he makes. He’s even had his bins raided and was castigated for failing to recycle a leaflet about children’s toothcare.

“I’m not a saint — may I make this absolutely clear,” he admits. “I am not running for sainthood. Politicians are human beings. But I’m trying to do better, so we’re trying to recycle more; we’ve switched to a renewable electricity supplier. We take the train when we come up here, but I’m not a saint. Shock.”

It turns out his baby son uses disposable nappies (reviled by the green lobby) and — whisper it — he doesn’t really believe in organic food. “It’s a lifestyle choice,” he says, dismissively. Healthwise, “there isn’t any evidence either way that’s conclusive”.

This won’t please the organic food industry one bit. But it’s a surprise and a relief to find him so frank — for Miliband is believed by many at Westminster to be the ultimate policy wonk, incapable of talking like a normal human being.

He used to head Tony Blair’s Downing Street policy unit, where he was considered so clever that Alastair Campbell nicknamed him “Brains”. I hear that on the football pitch — he plays for a parliamentary football team and is an Arsenal supporter — he calls players guilty of clumsy passes “clots” — an expression I last heard from the prim geography teacher at my all-girls’ school in the early 1980s. But he believes he’s got better at getting his message across simply since being in the cabinet. “It’s something that comes with experience, I think,” he says.

Miliband, 41, divides his time between his home in Primrose Hill, north London, and his constituency home in South Shields. It’s a good job for him that the taxpayer foots the bill for this weekly commute, for while taking the train helps save the planet (creating far fewer carbon emissions than driving or flying) the price of the ticket (£317 first class and £200 for a flexible second-class return) would put it beyond the means of most.

Miliband admits it’s extortionate but, disappointingly for his fellow commuters, it doesn’t sound as if he is about to step in and reduce fares to encourage public transport use.

“If you want French-level fares, you have to put in very much larger subsidies than we are at the moment. You’ve got to make a choice — is the taxpayer going to pay for it? Or is the consumer going to pay for it? I’m not standing here saying my top priority is to increase subsidies for the railway industry. That would be an odd thing to suddenly announce. Does price matter? Of course. But if you book in advance, you can get it cheaper. The government has got to decide the appropriate level of subsidy and since I’m a member of the government I obviously think we’ve got the right level of subsidy.”

It’s the sort of careful response you’d expect from a loyal minister. Not for Miliband the extravagant public attacks of Ian Pearson, his deputy, who last week labelled Ryanair “the irresponsible face of capitalism” and Michael O’Leary, the company’s chief executive, “just completely off the wall” for threatening to boycott the inclusion of aviation in the European Union’s carbon trading scheme. Miliband simply says that airlines should “play their part”.