From my experiance , I have found that the DD travel is god for my use, with that I have got a home made underquilt made from a Single Sleeping bag from Tesco's and been Toasty warm all night, I like the under quilt as it gives youplenty of scope, with it I have so far this winter been using my army Artic bag, or just using that as a quilt on top, last weekend I just had it thrown losely over me so cool air could get in.
As far tarps go, I have a 3m x 3m, and find it a tad small, I will be buying a 4.5 x 3 soon as i like the idea of the extra space, and that fact you can close the ends in with out compromise on space, this winter i closed the ends in on the 3 x 3 and found it a bit small for my liking.
A couple of pics for you, may give you an idea for your under quilt