well, if money is not a major issue, and the house is cold, just turn up the thermostat.
We have our thermostat set at 20 degrees, and have the heating on 24 hours a day, and the house is always boiling hot (wife likes it like that).
13 degrees sounds crazily cold.
i am doubling up the radiators in the next few days, cavity wall in a couple of weeks and a new front and back door in a week or two. im hoping they will more than cure our problem...
we have a little one incoming in 26 days, so its a bit of a rush