You could try getting someone to insulate outside of the outer box (with the correct insulator - rockwool etc. NOT fibreglass). We had this done recently and it's made a fair bit of difference. It also takes a fair bit of fiddling with the controls to regulate fuel usage, there may be a high permanent draft so you can adjust the "fixed" air supply slider.
If you're anywhere near the South East I can give you the number of the guy who fixed ours, highly recommended.
SSE