... They want me to sign the letter on the previous page. Should I?
IMO, definitely not.
The emissions figures stated when you made the decision to buy form part of the vehicle description. By signing the letter you are tacitly agreeing to a change in that description and hence may limit your right to redress.
...I really want my new car - but I'm worried about it hurting resale values in the long term (I've gone for the HP option as it was through drive the deal) so future values is a consideration for me...
As a private buyer that's the concern, the downside is potentially quite large, Glasses figures are down 2%, and that only covers October's figures before this debacle really got going, looking at local dealers there's some significant discounting going on, 5% seems typical and reducing the purchase price is one way of insulating yourself from that risk.
Run your figures through DTD again to see if the same spec vehicle is still being priced the same. If they are discounting further then you may be able to squeeze a few hundred quid from their dealer, not much but it might make you feel better?! Suppliers often try to hide any discounting as it quickly becomes regarded as normal, tweaking finance etc is a bit more discrete.
There's no requirement to sign the letter, it's at the dealers discretion.
Well the dealer said the letter doesn't affect my statutory rights...
Fine. If he's said it then he won't mind stating it on the letter if he insists you sign it?
If he refuses to do that then you've got your answer.