If you have a big oil heater it should still be able to heat the space efficiently - they throw out a lot of heat. It might also be something you have to be aware of over summer too if it is reverse cycle - my DH can't sleep with aircon on for the same reason.
Thanks Trill - our lounge is about 18 sq meters and then opens out onto the kitchen which can't be closed off - it's a pretty huge space so the oil heater doesn't cope with it - the other prob is I can't have it right where we are as can't have Blue rolling into it and burning himself and they get pretty damn hot....