My plan is to fit a trv to the hall radiator and turn it down,
along with the other room radiators.

This won't work. If the hall TRV is set above the room thermostat, than the
TRV will never operate. If the TRV is set below the room thermostat, the
heating will never turn off and is the equivalent to shorting out the room
thermostat. This will keep the conservatory warm. However, the boiler will
remain on constantly, even when no heat is required, costing you a fortune
if the boiler cutout doesn't operate.

The only solution that will really be satisfactory in your situation is to
have a separate zone for the conservatory. This means that the conservatory
will have its own programmable thermostat totally independent from the rest
of the house. It also allows different time scales to be used. If, for
example, the conservatory is used as a dining room, the timer can be set to
only heat it for the early evening, saving you loads in keeping a poorly
insulated conservatory hot for the entire day and evening.

Another alternative is to have TRVs in every room and remove the room
thermostat. Then an automatic bypass loop is used (any old manual one
removed) with a flow switch that can turn off the boiler when the automatic
bypass opens. This solution is not as good, as there isn't separate timing
for the conservatory. However, it may be easier to implement if the pipework
isn't conducive to sub zoning.

Christian.