JohnD,
I searched again and found that I got the TRV information from www.miketheboilerman.com. He has a heading entitled Thermostatic Valves.
Yes, I see he says "Another (technical) drawback is that they need the boiler to be on all the time making hot water available for the TRV to draw as and when it is needed. This rather defeats the fuel efficiency claimed for them in my opinion."
I'm a bit stunned, if he really means what he appears to be saying, that he doesn't use a room stat so he has the boiler running even when no heating is required. It's not a technical drawback at all.
It would be much more accurate, and simple, to say "the boiler needs to be on whenever heat is required from the radiators" (doh!) and, obviously, the boiler does not need to be on when heat is not required from any of the radiators (double doh!) so a room stat will achieve that, simply and economically.
