Greetings from Northern Ireland (ambient temperature 8 degrees C).
Fuck fleece blankets. Get a decent 4 season "mummy" style sleeping bag, and an air mattress to sleep on. I have a big fat Thermarest self-inflating mattress and a Mountain Equipment sleeping bag. $250 all together but this is a bullet proof
solution, unless you get it wet. I can pull the bag shut until only my nose sticks out the top and it has me warm and toasty in temperatures as low as 3 deg C (35 degrees F) in the back of my ambulance. I also wear a beanie while sleeping to minimize cold to my head and keep light away.
Any sort of blanket and you will have to deal with cold air getting in.
[I agree with Stefan's advice. When living in the Toyota Van, I also used a mummy style sleeping back and was never cold at night. When it got really cold (down to 7 degrees F a couple of nights) I wore thermal underwear, a wool cap, zipped up the sleeping bag and stayed toasty warm! Also note that some sort of thermal pad is very helpful (Z-rest, etc.) as is a cot or bed. The air barrier between bed and floor is the best insulator of all. --Skald]