A couple of Amiga A500 power supplies used to do the trick back in the day, nice n toasty
When I lived in Nottingham I didnt have central heating (had gas in front room tho) so I installed 400w slimline panel heaters in all the rooms, fitted them with timers and they certainly worked. Made by a company called Laser or Lazer and sold by B&Q retailed for around £35 each.
http://tinyurl.com/yr8m7g
good prices from the link ^