how many outside walls does it have?
Is the ceiling insulated properly in the loft (no gaps)?
Is the window draughty where it meets the wall (this is common with badly-fitted replacement windows) or where it opens?
Are there any holes in the ceiling, for pipes, downlighters etc?
Is the window misty in the morning?
Is there a draught round the skirting (also may leave black dust lines on the carpet)?
What is the size of the room, and what is the size of the radiator?
How old is the house and what is it made of? Are you in Penzance or Aberdeen?
If you want to use an electric heater, a small oil-filled radiator, fixed to the wall, is safest. Other types are a fire hazard if curtains, paper, clothes or bedding fall on them.