What type of free standing room heater provides the most cost efficient heat for a room?

I ask because our oil central heating has not really provided significant warmth in the cottage, mainly because it is a very old building, with draughty doors and double glazing on one side only.

Fiona will shortly put temporary secondary "double glazing" in place for the winter months and we know from experience that this makes a significant difference.

She is moving about for most of the day, either working in the garden, out and about, or snuggled under the duvet whist working online.

By contrast, I am sitting 9 to 5 and longer in the Rat Room, which is our small home office. But it is very cold, even having had the central heating on all day.

Heating the house all day just for me is a non-runner. Far too expensive and would not keep me warm anyway, so we intend to buy a room heater for me.
What do you recommend? I do not need blazing warmth, and am happy to wear four layers plus my cashmere beanie. I probably need just an increase of five degrees centigrade to help my brain slip into gear for my working day. But which type or model do you think is the cheapest to run?