@Lampedsomeoiks
I've had a quick read of this on the web. Basically, if you want the boiler to operate at maximum efficiency, you need the boiler to operate in condensing mode, which means using lower flow temperatures (temperature that the water is heated up to in the heating circuit).
If you switch off radiators in unused rooms, then the flow temperature will effectively be higher for a similar initial setting. So as you turn off the radiators, the flow temperature in the remaining ones will increase and this will limit the boiler efficiency.
So say for example you have 10 radiators on circuit, and your boiler is setup to a flow temperature of 70 degrees, which will give a higher level of condensing efficiency, when you turn off 2 then you will get a flow temperature of 80 degrees, which will give a lower level of condensing efficiency.
So you if you are removing radiators off circuit you really need to make sure the initial flow temperature is adjusted to something lower in order to get the best efficiency.
I might try it, because I didn't know about it. Unless you want to spend hours fiddling with your boiler, it's probably best to discuss it with your engineer when they service it.