For anyone interested in the above, here’s the answer. The plastic of the flow meter, or “topmeter” has split just above the thread. When I tightened it to try to stop the leak, the thread sheared off entirely and the water gushed out. When trying to unscrew the unit it just spun round because...
Did you ever get this sorted out, and if so, how did you remove the flow control in the end? I have the identical issue, same age of system, same type of valve. It seems to turn endlessly as if the thread has worn away and won’t come out. I’m tempted to force it as it’s going to have to be...