As seen on TV: Will David Cameron also get an Andrew Marr grilling?

When Andrew Marr steered last Sunday's interview with Gordon Brown towards the Prime Minister's health, he was arguably raising a point of genuine national interest. But when he asked if Brown was "one of those people" who relied on painkillers, as had been suggested – with no substantiation – by a little-known Tory blogger in France, and whipped up enthusiastically by his detractors elsewhere, Marr was entering dangerous journalistic territory.

For some, it was a legitimate question – if the PM is hooked on prescription medicine, we should be told. Former New Statesman editor John Kampfner leapt to Marr's defence, rightly welcoming a transgression of the usual love-in between politicians and lobby hacks. But for others, Marr was wrong to give legitimacy to a flimsy blogger's rumour on national television. As one Labour candidate, Alex Hilton, said: "Marr knows the difference between a rumour and a story. The difference is two sources."

Soon questions were being asked about Marr's motive. After all, he already had his answer, as Downing Street had earlier denied the allegation off-air. But as the husband of Guardian columnist Jackie Ashley and someone who was once known as "Red Andy", Marr could be keen to silence charges of being soft on Labour. With one well-timed tough question, Marr was perhaps hoping to put his impartiality beyond doubt.

Others got a whiff of humbug. Only last year Marr served an injunction on the media, forbidding any reporting of a personal story about him. So strict was the order that even mention of the injunction was an offence, until Private Eye successfully challenged this part of the order.

As the week went on, demands for an apology grew. The BBC has received more than 100 complaints. Marr has defended his actions. All eyes will now be on his interview with David Cameron today ahead of the Conservative conference. It is known the Tory leader smoked cannabis at school, and he has often been asked whether he snorted cocaine at Oxford. But if Marr fails to ask about drug abuse over breakfast this morning, we'll sure want to know why.