Ladies - Need help and info on Slimming Pants/Magic Pants. Please !?

I have a school ball coming up and have brought a beautiful black dress with a sort of band under the and the skirty bit of the dress flows out from there to just under the knees. I tried it on in the store (Monsoon) and it looked fine, but, I tried it at home and my stomach stuck out under the band and I look a little pregnant ! I am a size 16/18 and am hoping to get some of those pants that Gok Wan is always on about to flatten my stomach. I considered the Trinny&Susannah Belly ones but I think I need something right up to under the . Please help me out. I am open to ordering off the internet, but hate paying for delivery. Any stores which I can try ? And what should I go, which isn't obvious and flattens me out and makes my dress look as fab as it should ?