A fascinating day before the foreign affairs select committee which boiled down to the fundamental question about this Government: how much of its claims do you believe?

The Government's director of communications Alastair Campbell accused the BBC of lying when it said that the September 2002 dossier had been "sexed up" by the addition of the unsubstantiated claim that Saddam Hussein could attack with weapons of mass destruction (WMD) within 45 minutes.

Mr Campbell demanded an apology from the BBC because the 45-minute warning came from the intelligence services.

Mr Campbell said the BBC - once the bte noire of the Conservative Party but now a thorn in Labour's side - was so desperate to prove that the Government had dragged Britain into an unjust war that it was trawling for untrue allegations.

Perhaps we can believe Mr Campbell. Perhaps, even though the 45-minute claim looks increasingly incredible given the failure to find any WMDs, he is more spinned against than spinning.

Yet the same committee questioned Mr Campbell over a second dossier - "the dodgy dossier" - issued in February. This contains "evidence" found on the Internet in an American student's thesis that was more than 11 years old.

Mr Campbell admitted it was unfortunate that the source of the evidence hadn't been credited. But he said this accidental oversight was unimportant because the evidence was accurate.

Yet the evidence was presented to the public - who were, at the time, sceptical about war - as if it came from a daring secret agent who had risked his life in the desert to find out the truth. If the public had known its provenance, they might well have felt that the Government was so desperate to prove its case for war that it was trawling the Internet for anything that appeared to back it up.

So if Mr Campbell can happily preside over one trawl, how much of his claims that he is the victim of another trawl do you believe?

This is the Government's Achilles' heel. It so often manipulates information for its own ends that the public becomes sceptical of everything it says.

Now we question the justification of a war against WMDs - even while British soldiers are dying prosecuting that war.

There are genuine positives happening in education and the health service; there were genuine reasons for going to war. It is just that through the over-use of spin, the public finds it increasingly hard to know when the Government is telling the truth so it questions everything.