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Neville,
I would give very serious thoughts to installing a circ pump (even just temporarily) on the cylinder coil where it returns into the boiler. You will then be more or less certain of proving that the fault is with the coil design, even if you just power it up while the boiler is firing to do so. If contemplating this, I would suggest a Wilo Yonos Pico 6M (or 5M, but a bit of a odd ball) simply because you can set the head on this down to as low as 0.1M (I will check my own later to see), It is not ultra expensive (~ £125) to prove your point.
On your present System: Does the boiler just fire up when hot water only is required and if so what stops the thermosyphon effect on the upper rads on the pumped circuit?, is there a anti syphon valve fitted?.
Yes the boiler is called by the boiler thermostat when central heating is not called. No anti-syphon valve that I am aware of. I assume the stationary pump prevents thermosyphon to all radiators except bathroom.