Thank you all for your inputs.

My ECU failed late last week and I checked the off-charge voltage of a pack I removed from my other Hawk 10.4 but that combo has been flown without problem. !

I plugged in the JetCat GSU to check the voltage of the pack in the model where the ECU failed, 0 !
So it seems that if the ECU shorts, that short stays even if the radio is switched off and flattens the battery. The pack will not charge now as it is still connected to the ECU. (wings on, difficult to access). I will remove later today and check that the battery recovers OK.

Regards, David.