On Sat, 16 Mar 2019 16:41:54 +0000, Davidm wrote:

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?

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.