Eventually traced this to a dead current sense resistor. It's a 4 pin PowerTron, cost a small fortune £10. Somehow when the pin fell off or when I soldered it back on the inner "sense" output fried.
So, when running a 1A load I get
Pins
1-4: 100mV drop, continuity beeper, close to 0.1Ohms
1-3: 100mV drop, continuity beeper, close to 0.1Ohms
1-2: 0V, no continuity, OL Ohms
2-1, 2-3, 2-4: No continuity.
Pin 2 is toast.
Not a big deal, I just soldered it to pin 1. It's a Vishay powertron, no mention of fusing or anything in the DS. Also I never took it higher than 16A or close to 15W heat, unless the LiPo sume how surge currented it enough to have melted the solder on pin 2, but I expect that's unlikely, seemed more like the sense pin falling off was what drove the Mosfet control to slam the gate wide open in the first place.
http://www.vishaypg.com/docs/64238/64238_FPR_4_T220_T221.pdf