Well, the questions hasn't actually been answered, but then I wasn't really expecting a helpful answer.
I have been doing more research since posting the original question, and perhaps you could provide a more helpful answer, to you and also to me, that perhaps micro zoning using sTRVs is not going to reduce the heating bills, but correctly modulating the boiler, which only Nest currently supports out of the current selection of Smart Thermostats, definitely will.
The argument against micro-zoning can be summarised something like this: modern condensing boilers increase efficiency as the flow temperature is lowered; micro zoning increases flow temperature as the heated rooms lose heat to the lower temperature rooms, so the radiators have to maintain a higher temperature to compensate.
This is also why using a modulating thermostat, smart or not, with a modulating boiler saves money; the boiler only needs to run at a power to match the current heat loss, and not at 100% as would happen with an on/off thermostat.
I still need to find a way to quantify the saving in currency, but it is still a good point to differentiate Nest from other smart thermostats.
