This is what I have understood...
The "Downing Street Memo" is not a memo at all, it is personal notes taken by some observer totally out of the loop... In fact these "memos" do not even exist, the originals were destroyed... they were "re-typed" by God only knows who and the press is treating them like Dan Rather treats his "documents."
The 'memo' was not destroyed, one of the copies was leaked to Telegraph reporter Michael Smith who photocopied it and it was returned to it's place.
He has said that he transcribed the contents from the photocopy and destroyed the photocopy, I think that is where the misunderstanding comes from. There is no doubt that the contents are genuine however.