I looked at this recently. Basically all electric heaters are as efficient as each other (electricity in equals heat out) so I just got the cheapest one from Argos. Costs about 70p an hour to get the room up to temperature and then about 10-15p pet hour to maintain it for me.
Is there a forum-approved, energy-efficient, plug-in heater for keeping one room toasty for a couple of hours without turning the central heating on?!