Present proposition is to add three wall radiators to the dwarf-walled
conservatory and run these off my mains gas wet central heating system.
(...)

OK, but replace the timer with a programmable thermostat. This mean that you
needn't bother with TRVs and you have much better control of temperature.
Finally rather than radiators, consider a fan convector. These are superb
for conservatories because they are a fraction of the size of radiators and
can heat the room up in a fraction of the time, making it very suitable for
an hour boost etc.

I would recommend a Myson Hiline above the door, so it doesn't waste
valuable wall or floor space at all.

I wouldn't bother with the electric heater. However, if you are worried
about frost protection, stick a frost thermostat in there which turns on
your boiler/pump. Almost any programmable thermostat will have built in
frost protection and will already be calling for heat for the conservatory,
so the zone valve will already be open.

Christian.