[Neil]

Kate, you could also turn the radiators to almost off in the room you only want aired. Turn the valve all the way off. Then turn it on 1/4 turn. If this is not warm enough try another 1/4 turn until you get the desired effect. Not as good as a thermo valve but better than the heating being full on in rooms you don't want.

As for the heating not working without the water on it will not matter that much. Once the water is as hot as it can be it will only take heat from the boiler as the water in the cylinder cools. With a good insulation jacket this won't be too much. When you have a thermostat on the hot water cylinder you can set the hot water temp so save energy by not having the water hoter than you need it to be. In your case you probably set the hot water temp by adjusting the temperature on the boiler itself.

I think turning the boiler temp down will use less gas overall but it may take a little longer to warm the house up ( and your hot water will not be as hot )