Combi Boilers: what is more energy efficient?

The two previous are spot on, if you turn down the boiler and turn up the trvs the boiler will have very long firing cycles whereas if you turn up the boiler and set the valves to your comfort level in the respected rooms and close the room doors to keep that heat in then once they have reached the desired temperature those valves will close and reduce the load on the boiler and shorten the firing time so costing less. To improve system more get a room stat fitted and they can be done wirelessly(apart from the reciever connection to the boiler)