Clive,
I've gone through your flow explanation and appreciate it.
I'd like to add, ref
If I have it right, when you light up the afterburner keeping the primary jet exit area the same...
From, for example, "Fast Jets - the history of reheat development at Derby" by Cyril Elliott "... in order to keep the mass flow constant, the area of the nozzle must increase with the sqrt of the jet temperature." (Since gas specific volume has increased significantly).
This book incidentally probably holds the world record for foldout size, 66" for the Adour reheat fuel system schematic.