I just bought an electric frost watcher heater similar to this. http://www.machinemart.co.uk/shop/product/details/fsw505-frost-watcherTurn the stat down to it's lowest and it cuts in and out all night as needed. You've only got to keep the temp above freezing, and with a bit of insulation as well it works fine. And at 400 watts its not gonna cost the earth to run.