William Hague's opening question had been a request for clarification about Labour's "year of delivery". Could Mr Blair confirm that the latest figure for the fall in police numbers since the election?
Mr Hague delivered this in a tone which he is fond of - his voice polite but a little puzzled.
It is the tone which a loss adjuster might use when questioning a policy- holder's claim that the valuables missing from his council flat include two Faberge eggs and a Titian drawing. It makes Mr Blair very tetchy. He naturally didn't supply the figure Mr Hague had asked for, which allowed the Conservative leader to read it into the record before moving on.
If Mr Blair had difficulties with the first number, he said, as the Prime Minister fidgeted uneasily opposite him, perhaps he could help him with another little niggle he had. How many more people were waiting to see a consultant since the last election? Mr Blair let his edginess show.
"Actually," he said querulously, and then he paused. Tory MP's happily filled this useful gap with mocking laughter. "Actually" is one of Mr Blair's most regular verbal tics. A word he finds indispensable in confronting what he sees as malignantly prejudicial accounts of New Labour's Long March.
"Actually" spoken with a kind of weary exasperation, is usually the preamble to a corrective burst of counter-statistics. It economically sketches a world in which Tory fantasies stand opposed to Labour verities, in which Mr Hague's gloomy delirium has to be contrasted with Mr Blair's clear- sighted realism.
"Actually" epitomises the Prime Minister's deeply felt conviction that he is never wrong on a matter of fact and he uses it so often it is difficult to know what he would do without it.
Tory backbenchers decided it would be fun to find out. Their mood was already buoyant, since they had been treated to a textbook demonstration of parliamentary craft from their leader. Mr Hague had arranged his questions in such a way that they depended on Mr Blair not answering them.
Rather than returning to the hopeless task of pressing a straight answer out of the Prime Minister he would follow up with a brisk dismissal of the answer he had received and then move on to another damning statistic.
Mr Blair didn't know what to do first - dismiss the dismissal or parry the new attack - and the result was that he looked flustered and unable to keep up. Tory MPs roared approvingly as he broke off from answering one question to improve his reply to the previous one, since this air of divided attention was precisely what Mr Hague had been trying to achieve.
Mr Hague's troops began to cheer in an appreciative way every time Mr Blair used the word "actually". This was, like many of the most effective Parliamentary tricks, rather childish.
There was the Prime Minister - a short-tempered teacher haplessly trying to drum some elementary knowledge into his pupils, and there were the pupils, cheerfully undermining his authority. Mr Blair began to look a bit queasy. He knew that his favourite word was out of bounds, but to avoid it would require the concentration which he should have been devoting to plausible denial.
His plight brought to mind that game in which you have to thread a metal loop along a twisted wire without setting off a buzzer.
The Prime Minister didn't make it - with the Tory backbenches going off loudly on several occasions, sometimes loud enough to make him jump. He probably thought it was "pathetic", another word he favours. It was extremely effective, actually.