True , Nialla, I didn't cosider the "gun pipe above the open fire" to be a central heating boiler. Modern solid fuel boilers (those that are build within the last thirty or forty years) come with overheating protection, as much as gas or oil boilers.
Even the old backboilers have an overheating protection, the expansion tank. Leaving a radiator open is no safe overheating protection. A radiator would deliver/let out only a certain amount of energy. If the backboiler produces more heat than the open radiator lets out we would be where we started - without a safety rope. Therefore the expansion tank is attached to the open backboiler system as a legal must.
If it was dangerous to run the backboiler system without a valve open on one radiator then there shouldn't be a valve fitted to at least one radiator.To avoid accidents.This legislation-that one radiator should be without a valve- does not exist. Because it is not necessary. The safety aspect is fully covered by the expansion tank.
So there is no need to keep a radiator without a (thermostatic)valve.No matter what boiler is delivering the energy.