We have really good heating but in the depths of winter we pull out a portable oil-filled radiator. It runs off electric and heats up the oil inside the radiator. Once it's hot, the thermosat knocks off but the oil still radiates out heat. Once it gets cooler, the thermosat kicks back in and heats it up again.
We have it on alot and don't notice that much of a change in the electric bill. You never need to top it up or change it. Our costs about £50 I think.
Here's some on Amazon. It's great because you can move from room to room, there's no chance of fire and there's no carbon monoxide.