Have you noticed the Jeremy Corbyn sigh? It comes out whenever he is asked a question he doesn’t like. There is a pause, an impatient expulsion of air before he addresses whatever impertinent question he has just been asked.

The sigh hasn’t been spotted yet today, so far as I know. But the Labour leader has plenty to sigh about after the result of the Brecon and Radnorshire by-election. His side garnered only five per cent of the vote in a tight contest between the victorious Lib-Dems and the Tories.

The good news is that the Tories have seen their Commons majority cut to one. But Labour came behind the appalling Brexit party, more of a Nigel Farage pyramid-selling scam than a true political party. Five per cent of the vote; in Wales! Heavens that’s gloomy.

Let’s return to that sigh. It was spotted in Westminster during Boris Johnson’s first PMQs when Johnson started puffing and blustering and being a pompous buffoon. Corbyn sighed and looked ratty before he responded with a series of questions Johnson declined to answer.

This might seem a trivial matter, but is Jeremy Corbyn’s sigh enough of a weapon against Boris Johnson’s absurdist bluster? I fear it isn’t up to the job.

To the non-political obsessive (ie most people), Johnson is fun and a bit of a character, whereas Corbyn looks like a geography teacher who is disappointed in your homework. Yes, Corbyn was inspirational during the election he nearly won/actually lost. But that was yesterday’s tussle, and the next one will be different, especially with the revived Lib-Dems and the bothersome Brexit party complicating the picture.

Labour’s polling is appalling and Corbyn’s personal ratings are terrible. Still, the faithful will stand by their man. They will repeatedly share social media posts about how he has received the worst media treatment of any politician; they will insist that the antisemitism scandal is a put-up job engineered by Corbyn’s enemies.

Maybe the Corbyn ultras have a point, certainly about the hostile media. But the newspapers have mostly always disparaged Labour leaders (apart from a flirtation with Tony Blair). Yes, Corbyn has had even more aggressive treatment than usual, but in a sense that was always going to happen. As an ageing, unreconstructed leftie, he’s an easy target, a perfect hate figure for the right-wing press.

People vote for important reasons or shallow reasons. Sometimes they vote because they ‘like’ someone. Boris Johnson is such a Marmite politician than even jars of Marmite can’t make up their minds. But he has a robust personality: a scheming, truth-bending put-up job of a personality, but his act might fool enough people.

Jeremy Corbyn’s sigh should worry anyone who despairs at the prospect of Johnson winning an election, rather than being handpicked in a disgraced Tory beauty contest.

Johnson’s hard Brexit cabinet may be a deeply dispiriting band chosen exclusively for their willingness to go for a no-deal Brexit if necessary.

But Brexit remains the endlessly tedious, ticking time-bomb of the day. Corbyn has shown no willingness to disarm that bomb. His long game plan has always been to wait for the Tories to mess up so he can win an election. Along the way, his inconsistency over Brexit (does Labour have a firm policy; does anyone have a clue what it might be?) has undermined his reputation as a straight talker who tells it like it is.

Brexit is toxic all round, unless you are that appalling Farage man. But that toxicity is harming Labour.

Jeremy Corbyn needs come up with a way to puncture the Boris balloon. His sigh alone isn’t sharp enough for the job.

These thoughts are offered not in hostility but in a mood of quiet despair.