Central Heating Balancing

No, you dont need a flow control valve. You should have a bypass valve, or, one of the radiators without any TRV control. Thats so if all radiators have reached their set temperature, the TRV shuts, stopping flow through that raditor. If all rads have TRVs, and all are shut, there is nowhere to go for the heat output from the boiler.

The boiler will only come up with an overheat fault if there isnt enough flow to keep the heat exchanger output below the recommended temperature (70 degrees?), or, there is a fault with the boiler, or a restriction somewhere.

So maybe the overheat error occurs when all the TRVs are closed. I have always thought that a programmable thermostat was a better arrangement.

As I said in para 1 of my OP (which isn't in your reply) I have one rad with no TRV for this very reason. I also have an auto bypass valve, one side connected before the two (CH and HW) zone valves and the other side to the boiler return.

So why don't I need to control the flow in the HW circuit, and balance that out with the flow in the CH side?

Better than what? You fit the stat in the coldest room, usually the hallway which also has the rad without the TRV.

I have a progammable thermostat, and one rad with no TRV (see my original post).

It is an S plan, the HW flow will stop once the hot water is at the correct temperature. Balancing is to get the radiators to all achieve the same temperature, or, adjust them if you want one hotter than the others. It is done to make sure that the first radiator(s) in line do not take all the heat.