The Prime Minister has asked me to thank you for your recent letter.
Mr Blair would like to reply personally, but as you will appreciate he receives many thousands of letters each week and this is not possible.
The matter you raise is the responsibility of the Department for Constitutional Affairs, therefore he has asked that your letter be forwarded to that Department so that they are also aware of your views.
Or in other words: 'The prime minister hasn't seen your letter, when we say he asked that it be forwarded we didn't mean your letter precisely, but rather that he left an instruction not to bother him with these things. In this way he can have the honestly held belief that people aren't concerned with this issue'.
Or is that overly cynical?
Still listening, Tony?
P.S. I'm not naïve enough to assume that he would reply personally, but I do resent the implication that the PM has read the letter without actually saying he has explicitly. If he has read a letter include 'The Prime Minister read your letter and asked me....', if not, use the above form to give that impression without saying so..