We recently came to the conclusion that it's a false economy to hold out, as once the walls cooled we started getting a damp issue, which would only heat loss. As autumn is the worst time for humidity, we have shifted to putting the house on a thermostat (18 degrees upstairs, which so far has triggered only once this autumn). SLTD, if you do go for thermostatic valves on the radiators, be aware that one rad must always be left without one, usually the bathroom if you have one there. I can't remember the exact reason, but it has to do with the main thermostat telling the boiler to heat, but the radiator ones telling them not to heat, and the tank therefore overheating or something like that. Also, whilst I can in no way predict the behaviour of baby SLTD, I have spent the past three winters re-adjusting the valves after mini Squelchy has gone through and changed them to whatever number is his current favourite. This, at least, is better than the time we called the plumber out because we had no hot water only to find that this too was controlled by a tempting little dial at toddler height that we hadn't known he'd spotted.