You do need the time switch as you cannot rely on the boiler shutting down when all TRVs are shut. You should not have a TRV on the radiator in the room where the thermostat is. The TRV can close before the thermostat removes the call for heat. The boiler may then have a demand for heat but no flow.
Some boilers require that a bypass valve id fitted to the pipe work. The bypass valve opens if the pressure from the pump suggest all TRVs are shut. The flow is then via the by-pass. This is necessary to allow the boiler to cool down after there is no more demand for heat.
But if the by-pass is fitted then it is possible for the boiler to continue to run heating only the pipes to and from the by pass. The boiler will shut down if the returning water is too hot but the pump keeps running, the water cools and the boiler fires up again.
