Do you have a thermostat in the living room too? If not then the boiler will keep running until the thermostat upstairs (in the hall?) senses that it is at the required temp. TRV's on the other radiators would help if there's no other thermostats.
Have you got thermostatic rad valves and is the system properly balanced? In my opinion room stats work better sited downstairs but that doesn't necessarily mean yours won't do the job where it is.
As said thermostatic radiator valves on the radiators will help keep the rooms at a more comfortable temperature.
How long has this been a problem?
System may require balancing if other rooms are not upto temperature.
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