Theresa May must do more than just say she’s not Jeremy Corbyn | Gaby Hinsliff

Once upon a time, there was an unelected prime minister who rose into the job at least partly through sheer force of personality. This leader faced no real contest, because there frankly didn’t seem much point; there was only one obvious grownup in the room, whose record seemed to speak for itself. It was only after several months that people began to ask just how much they had taken on trust, and whether they had confused great moral certainty for something more.

But enough about Gordon Brown. Last night it was Theresa May getting the once-over from the BBC’s Andrew Neil, fast emerging as the interviewer no underprepared politician wants to meet down a dark alley. It did not go tremendously well.

The trouble with ascending unopposed to the leadership as May and Brown did, before governing in her case largely untroubled until now by a divided opposition, is that big ideas don’t get stress-tested enough, and nor do flaws in the way your team works. So it seems to have proved with the manifesto plan unveiled last Thursday to make people with conditions such as dementia fund seemingly unlimited amounts of their own care at home using the equity built up in their houses (although they’d be able to keep the last £100,000, and no home would be sold until after their deaths). And if you’ve had to read that twice to understand it, imagine explaining it to worried pensioners on the doorstep.

Hours before the BBC interview, amid a growing backlash against the so-called “dementia tax”, May had hurriedly announced there would after all be an “absolute limit” on costs to stop anyone paying too much. But this change from “no cap” to “actually, a cap” was not a change, she insisted. Even though the manifesto didn’t mention a cap on Thursday, and the health secretary Jeremy Hunt said specifically on Friday that it was being dropped. No, this was a clarification, prompted by terrible Labour “scaremongering”. That car, now driving the opposite way down the street? It hasn’t U-turned. It’s just clarified its direction. You did wonder if poor Hunt will be clarified right out of the cabinet once the election is over.

It’s possible that to those very many viewers who haven’t read the manifesto or was only half-watching while cooking supper, this interview sounded more plausible – or at any rate will by the time Neil has demolished all the other leaders in turn.

May still has the headmistressy knack of sounding calm and reasonable, dependable in a crisis. Her rebuff to the charge of producing an uncosted manifesto – that it’s usually Labour in the dock for that, not the Tories – was shameless but reminded me of something I’ve seen in focus groups; most voters barely follow the detail of a campaign, relying more on assumptions about what the parties normally do than on evidence of what they’re actually doing.

The danger is that all this undermines two of May’s big trump cards. The first is her claim to brutal honesty, to being the vicar’s daughter who tells it straight. She might just have got away with presenting a rethink on social care as an honest response to an honest mistake, proof she’s listening. But to spell out the tough decisions she believes are required on social care funding, then backtrack but swear she hasn’t, sounds anything but straightforward. The most telling moment came when Neil asked how her pledges could be believed when she repeatedly failed to deliver promised cuts on immigration as home secretary, and she answered that “I called an election on this whole issue of trust”. Well, yes; and that’s why this is a problem.

But the second damaged trump card is her reputation for brisk managerial competence, which draws many floating voters to her and is the reason Tory remainers are largely sticking with May; if Brexit is inevitable, they have assumed she’ll make a better job of it than Corbyn would. The Neil interview suggests she simply hadn’t grasped that in the middle of a wobble – even if it turns out to be a temporary wobble - “but at least I’m not Corbyn” doesn’t cut it. To secure a proper mandate she needs to spell out who she is, not just who she isn’t.

Voters still know little about May because of the unusual circumstances in which she became leader – everyone else self-destructed before she’d even had to set out her stall – and the fact that Brexit has drowned out other issues. Many have been willing to assume she knows what she’s doing even if she isn’t telling, but they won’t give her the benefit of the doubt forever.

And as with Brown, there are hints that all is not well inside No 10; that her habit of restricting everything to a tight inner circle famed for its territorial aggression is undermining good policymaking. Senior minsters apparently knew nothing about the social care plan until the last minute, which is no way to make (let alone publicly defend) complex and sensitive policy. If she wants to recover the trust she has just squandered, May must take others into her confidence – and that includes the voters.