Cleaned up the transformer, i.e. broke off the molten plastic under a running tap;
seems like the primary coil was hotter than the secondary- (see pic attached).

Question to the electric boffs out there is :
can such a transformer continue to supply power to the control panel
for a few minutes whilst failing itself?

Why the control panel stopped after two minutes before an AC Power-failure signal I'm not sure;
I found another bare circuit board which was probably the radio transmitter's,
(typically wall-mounted nearby outside the control panel's box), so perhaps it got nuked
(although I might be leading myself by the ear on this rambling hypothesis...)