A rubber backed picnic blanket under your self inflating mattress is the way to go.
The self inflating mattress should be the thickest you can afford.
Sleeping gear ? Again, get the warmest featherdown sleeping bag you can afford,if it gets too hot, you can always open it or use it as a doona.
Now the tricky bit, ditch your pj's, No, I don't mean you to sleep in the nuddy, (good idea at times though ) get yourself a pair of good lambswool thermals, use them as your night attire, you'll never be cold again.
I've found the best thing to do incold weather for a nice warm night, is ALL the above and before you don your super dooper thermals, have a nice hot shower, top it off with a nice filling meal, coupla red wines and crawl into the sack. First class every time.
HMMMMMM, what was that reference to 'hot shower' you may ask ? Yep, always take a shower system with us. There are many devious ways of concocting hot water for a shower, no matter which one you choose, it'll be well worth the effort at the end of the day, and IMHO, winter is the BEST time for camping.