It's one of those conundrums that doesn't have an answer - well it does, TRV's on all the radiators for starters and seprate zones. The main problem with a thermostat, not in the hall but half way up the stairs, as I had one once, was not the close control or the step change on front door opening (which never happens as, in real life most of the other doors are open and the cold air hits your feet) but having the hall in the north of the building in winter and sun heating wintery south facing rooms which then got too hot as the heating was on. The sitting room has problems as have been described.
In the light of experience, I would put the thermostat in the dining room if you have one. It is less likely to have separate heat sources or an outside door and is still a living room and reasonable furnished and curtained (usually) and only slightly cooler than the sitting room. It's not ideal but best you can do IMHO