Replacement controls for Myson Envoy (1977) fan convectors?

Where can I get replacement controls for a Myson Envoy(1977) fan convector?

I recently moved into a house that has a SIME boiler under the floor and several Myson fan convectors on the walls. Half of the fan convectors have broken control knobs. 2 broken thermostats and 4 broken fan controls.

Also, can anyone tell me how to control the boiler and fan convectors from a single thermostat? Setting each fan convector and the boiler with independent thermostats gets a bit tedious. The main thermostat is an old Honeywell unit with a dial to set the temperature.