There is always lots of talk of door draught excluders to prevent draughts, but a big cause of draughts are chimneys. If you have an unused chimney, then consider blocking it off or buy an inflatable plastic 'flu bag' which can be inserted into the flu and then inflated so it fits tightly into the flu and cannot be seen from the room. I did this just recently and was surprised that my house was still warm in the morning - usually it was very cold before the heating came on and the wind used to whistle under the front room door.