Both the letter and the questioning were part of the Public Bill Committee consideration of the Crime and Security Bill (and the Home Affairs Committee has also been covering DNA recently).
Question raised by James Brokenshire in Committee Stage: 7th sitting: House of Commons 4 February, 2010 Col.234 ish
though the answer didn't appear until after a Division break.
The reply from David Hanson was in Committee Stage: 8th sitting: House of Commons 4 February, 2010 Col.252
near the end of the page.
At least some of the details of the letter are mentioned.
Edit: Reading a little further, it seems that the letter is *not* classed as "evidence" because it was not addressed to the Committee as "evidence".
The Chairman: Order. I might be able to help. The letter was sent to Members. It becomes evidence only if it is sent to the Committee as evidence—if it is stated that it is sent as evidence. If that helps to clarify the matter and deal with the hon. Gentleman’s concern, we might be able to move forward.
Apparently the letter was addressed to the Chairman and co-Chairman - it gained effective status as evidence (and was considered in the same way) without actually being 'evidence' and nobody gets in trouble for it being wrong.