In fact, there appears to be two, seemingly independent, sources for the statement.
The better known is the evidence of David Broucher given to the Hutton Inquiry on the afternoon of Thursday 21st August 2003 at Page 145.
1 A. As Dr Kelly was leaving I said to him: what will happen
2 if Iraq is invaded? And his reply was, which I took at
3 the time to be a throw away remark -- he said: I will
4 probably be found dead in the woods.
5 Q. You understood it to be a throw away remark. Did you
6 report that remark at the time to anyone?
7 A. I did not report it at the time to anyone because I did
8 not attribute any particular significance to it.
9 I thought he might have meant that he was at risk of
10 being attacked by the Iraqis in some way.
The less well known is the statement made by Mai Pederson here: David Kelly's closest female confidante on why he COULDN'T have killed himself.
Mai Pederson comments on this, saying, "He also told me that if we invaded Iraq, he would be found dead in the woods.".
Of course, the question remains open as to whether David Kelly was referring to suicide or murder.
However, the existence of evidence suggesting that David Kelly said this to at least two people seriously undermines Lord Hutton's convenient discounting of the evidence of David Broucher.
Importantly, if one accepts that David Kelly told David Broucher in February 2003 that he would be found "dead in the woods" if Iraq was invaded it means that whatever the motive for David Kelly's death, it was apparent to him as far back as February 2003.
If that's correct, then looking for a Tom Mangold-type melodramatic telephone call on 17th July 2003 is overtly a red herring.