Don't know but that seems the obvious, quickest and simplest solution.
At the same time, using separate "police investigation" type interrogations, I would ask all the memos addressees David Manning, Defence Secretary, Foreign Secretary, Attorney-General, Sir Richard Wilson, John Scarlett, Francis Richards, CDS, C, Jonathan Powell, Sally Morgan, Alastair Campbell if they had any direct explanation or expansion on this comment from Rycroft.
Just to see if all the stories match up.
Really though, unless this is done all the discussion in the world about it is pretty meaningless. I'm sure it was clear to Rycroft when he wrote it. Why not ask him? He probably kept notes and/or drafts of it. Those might shed a bit of light as well.