Am just about to make my annual pilgrimmage to Go Outdoors before we go camping. Top priority this year is to keep warm at night. Is it worth splashing out on expensive sleeping mats or any other gear? I don't think I can afford Thermarests but might do if everyone says they are fantastic. Are they warmer than a cheapo mat and and airbed on top? Last year's sleeping bag from Tesco was a mistake so would rather buy some decent gear this time but want to spend the money wisely.