our problem is that the thermostatic valves on the radiators are stiff to turn and tend to be left open so that we end up with an office that is roasting hot. We have 4 radiators in total down stairs.
I could replace the valves heads but the valve for the largest radiator is stuck behind a desk and is clearly very inefficient.
I have looked at having a NEST fitted but not sure whether it would be worthwhile. I presume that if the NEST thermostat controls everything then I do not need thermostatic valves. Is that correct? The floor area is low enough that one thermostat could very well control everything I think.
I think you might have your own answer:
1. If you have a room thermostat already on the ground floor and you think this would adequately control the offices, then leave the TRVs wide open and let the thermostat control the space heat. You shouldn't have TRVs and room 'stats fighting each other to control the space heating.
2. If you don't have a room 'stat (and the boiler is using return temperature to control output), then replace the TRVs and set them according to the temperature requirements of each room. I'd be inclined to do a proper job and replace the body of the valve as well as the TRV "top". It's probably the body of the valve that's stiff not just the top. This will then give you reliable control if you use decent TRVs (I've used Drayton TRV4s in several houses with no problems, but cheapy ones have had a few problems over the years). Yes, this might mean moving a desk and a bit of temporary pain and expense but it's better long term than roasting the place and modulating the heat by opening windows in the dead of winter. If you're a gadget fiend then some sort of electronic TRV might allow individual room times/temperature etc but they are pricey and introduce additional maintenance issues (battery replacement at least every 5 years). In a commercial setting, you might look for TRVs that can be locked to a fixed temperature to prevent staff tinkerage.
Use the boiler timer controls to switch off or set-back the heating to a lower temperature overnight/weekends when the office is unoccupied.
I don't have any direct experience of Nest, Hive etc, but they sound like overkill for a small office. If you have timer control on the boiler and local control for space temperature then smart thermostats are a bit of an expense unless you're really going to use your phone/tablet to tinker with the heating remotely.