How do you prove a negative?

The revelations in Ron Suskind's new book (see e.g. the Times) that the UK and US were told before the war that Iraq had no WMD are fascinating. It's not clear that the claims of Iraqi intelligence chief Tahir Jalil Habbush should have been believed but where do they appear in e.g. the Butler Review?

Trying to work out the timing is quite difficult. The Times says:

The book claimed that the former Prime Minister sent a top British spy to the Middle East in 2003 — three months before the invasion

With the invasion taking place on 20 March 2003, three months before that would be 20 December 2002.

The other main claim from Suskind's book is that after the war the CIA used Habbush to forge a letter linking Iraq with the September 11 attacks. The Times gleefully says:

The forgery, adamantly denied by the White House, was passed to a British journalist in Baghdad and written about as if genuine by The Sunday Telegraph on December 14, 2003.