IT has been described as the moment when Boris Johnson's balloon burst, when his chances of becoming Prime Minister were dealt a fatal blow.

"You are a nasty piece of work, aren't you?" said Eddie Mair, stand-in host of The Andrew Marr Show. Had there been a studio audience you would surely have heard it gasp.

In response, the mayor of London looked like an old lag who'd hoped to be given community service but was now resigned to spending the rest of his days behind bars. For once, Mr Johnson's much-vaunted charm eluded him. He was dumfoonert, clean-bowled, his stumps scattered, out for a duck.

Spluttering and bumbling, he groped for a way to extricate himself but there was none. Mair, meanwhile, sat and said nothing, content the stiletto had done its job. If in the future voters ever have to consider Mr Johnson for PM they can be in no doubt of the calibre of contender.

For here is a man who lies and lies and lies again. Nor are his lies white ones. He makes things up then denies them. He insists he didn't do something he obviously did. And he was happy to indulge Darius Guppy, an old Etonian mate, when he asked for the phone number of a nosey journalist whom he intended to beat up. Mr Guppy, says Mr Johnson, reaching for his copy of Lemprière, lives by a "Homeric code of honour, loyalty and revenge". What code, one wonders, does Mr Johnson observe? The Code of the Woosters?

But let's leave the jackass in his midden and instead praise Eddie Mair who for some time now has been Britain's brightest broadcaster. That he is not as well known as ought is because he is to be found on Radio 4, where at five on weekdays he presents PM which is as entertaining as it is deeply serious.

Mair, who hails from Dundee, and did not go to university, is that most lethal of interviewers. Unlike, say, David Frost, he does not flatter his subjects in order later to skewer them. Nor, indeed, is he remotely like Jeremy Paxman, whose successor he is touted to be. Mair exudes bonhomie and repartee not petulance and arrogance. Moreover, he rarely interrupts, knowing this will only put interviewees on their guard.

There is, too, something about his Scottish accent which leads some people, such as expenses abusing MPs, unwisely to confide in him, as they used to put their trust in bank managers and doctors. But the trust Mair inspires is that which listeners and viewers invest in him. You never doubt for a moment that he is on our side, that the questions he is asking are ones we would dearly love to hear answered.

In that regard, he is the embodiment of what the BBC is supposed to stand for. Nor, it must be underlined, is he alone in metropolitan exile. There are, it seems, Scots in every burrow of British broadcasting. Thus, when Andrew Marr is unable to appear, Mair is his substitute. Or Allan Little, who, with his sonorous Gallovidian accent, often sounds as if he's reciting poetry rather than relaying horror stories from some hellish part of the globe. And then there is Jim Naughtie, from Milton of Rothiemay, that hothouse of broadcasting talent, who, if he's not careful, may shortly be declared a national treasure.

Nor are Scotswomen any slouches in front of the microphone or camera. In days of yore, Desert Island Discs was not a programme to rouse you out of Sunday morning torpor. But since Kirsty Young (from East Kilbride) assumed the mantle of presenter it has become much more compelling. As with the other Kirsty – Wark – or, indeed, Radio 5 Live's Asmir Mir, she does not shy from asking difficult questions with a directness that is as refreshing as is it disarming.

I could go on adding names – Andrew Neil, Alan Johnston, Sally Magnusson, Gavin Esler – to the memorial cairn. What defines them all is not just their Scottishness, which is self-evident, but their inability to obfuscate. They don't shilly-shally or beat about bushes or resort to euphemism. As BoJo has found out to his cost.