I wouldn't expect you to experience much heat loss from the pipes through which there was no flow. In my house, most of that issue would not arise, since zone valves close to the boiler isolate pipes in parts of the house not being heated. Like you, I have been heating just my living room in some recent evenings, and that has not involved hardly any pipework other than that from boiler to living room getting hot. How efficient a boiler is when only heating one room, I'm not so sure.Those are the ratings from the installation manual. I haven't measured the efficiency - to do that I'd need an accurate value for the rate of heat loss from the room per degree temp difference inside to out, which I don't have. However, even if I only want to heat that one room (which at this time of year is the case), if I use the central heating I have heat loss from all the piping in the house even if all the rads except those in the living room are turned off. The pipes in the first floor are not insulated, so I am heating the floorspace and thus, to some extent, the other rooms too.
That would be interesting. If you ever do it, do tell us the answer!I guess one way I could do a comparison is to measure the gas usage for one evening, and then compare it to the electricity usage (I have plug-in meter and the aircon runs on a 13A socket) on another evening with similar outside temperature and wind conditions.
Kind Regards, John