We have no central heating and no thermostat. Normal morning temperatures at the moment are around 11 degrees. The portable gas heater goes on one bar in the sitting room for most of the day which keeps it around 16 degrees. It's amazing how quickly you acclimatise - 16 degrees does actually feel warm now.
Our rule of thumb is, if we have two jumpers on and we're still cold then the heater goes on. If we're still cold then we grab a blanket.