So bewildering are the invisible forces governing PM's Questions that it's hard to grasp the latest arcane theory on yesterday's tumultuous exchanges on the Mitchell affair.

Which is that by calling the Chief Whip "toast" Ed Miliband has ensured he isn't. And that's good because it's better to have him as a permanent (groans all round) whipping boy.

Confused? You will be, to misquote the tag line of the cult Seventies sitcom Soap. Because Miliband appeared to give it all he'd got as Mitchell himself sat uneasily through the proceedings, separated from Cameron by his cabinet colleagues Andrew Lansley and Nick Clegg, presumably to ensure that the Prime Minister and Chief Whip did not appear in the same television shot. Just in case.

Anyone else would have been arrested, the Labour leader suggested. "While it is a night in the cell for the yobs, it is a night at the Carlton Club for the Chief Whip. Isn't that the clearest case there could be of total double standards?" (Great primetime product placement for the Carlton, the venerable high-Tory hangout Mitchell was hurrying to that fateful evening.)

Certainly the Tory benches seemed oddly restrained as Miliband pressed on relentlessly, only springing briefly to raucous life, when he continued: "If you are a millionaire you get a tax cut, if you are everybody else you get a tax rise." Like shipwrecked passengers lunging at a lifeline, the Government's supporters were (apparently) jeering at what they see as Miliband's reluctance to declare his own worth.

Yet the one moment when David Cameron became really angry was when Labour's Chris Bryant asked him whether his refusal to publish all the emails and texts between No 10 and News International – Rebekah Brooks included – was because they were too "salacious and embarrassing".

Maybe it was Bryant's blood-curdling, finger-pointing taunt: "I wouldn't smile if I was him. When the truth comes out the Prime Minister won't be smiling."

But Cameron seemed to decide in mid-flow on his no-reply strategy, convinced by his own eloquent expressions of outrage at Bryant for having previously read out embargoed "Levenson information", much of it "untrue".

"Do you know what," he said with startling suddenness. "Until he apologises I'm not going to answer his questions."

This is a first. Opposition whips are presumably combing through the list of more obscure and blameless backbenchers to ensure that tough questions in future are only put by MPs who have no prior cause to apologise to the Prime Minister.