Steve - what's your central heating run on?
Im only 1/2 way through it here, but i accquired a large metal storage cupboard (large enough to stack 4-off the 60ltr drums i collect, along with a few cubies of glyc for the wash) and a radiator. I can fit 25mm of insulation all over the inside (about £7 for a 1200 x 2400 sheet from wicks) and 50mm all over the outside.
I know that may be oversized for what you want, but, if youre running your heating on somthing cheaper than electric, it may be worth considering?