Ok with gas getting more and more expensive, i'm wondering if anyone can help me out.

I have a 3 bed semi, a fairly decent size one, not too small, not massive either. There's only me and my girlfriend living in it. Because we only need to heat the lounge and bedroom (kitchen heats itself from the cooking side of things) i'm wondering if its cheaper to use the cental heating as little as possible, basically just enought to stop the pipes from freezing and using my open gas fire in the lounge which would be on most of the time when were at home or would i better better off just wacking the CH up and forget about the gas fire?

This is from a financial view only. Trying to save a few pennies thats all. I'm a right tight git