Am I missing something in my thinking, before I order the parts?

1. You may need something to prevent gravity circulation in the CH circuit.
This will depend on circumstances, but could be either a 2 port valve or a
one way valve (with circuit flow set to be against natural convection). It
may not be necessary.

2. Remove the TRV from the roomstat room. Some well meaning person WILL turn
it down.

3. Ensure boiler is safe with valve on the gravity circuit. It may need that
circuit to be free flowing for two reasons. Firstly, it may need somewhere
to dump heat at all times (particularly solid fuel boilers). Secondly, it
may need the circuit to provide a steam outlet/quenching water inlet path.
This would either require careful layout to ensure that the 2 port valve
doesn't block those paths or require the boiler to have an overtemperature
safety cutout and pressure relief valve.

4. Consider pumping the hot water gravity circuit.

Christian.