Well for anyone following along in future, I took one panel apart:

The leds are arranged in two strings, one on each side, each string 7 off x 14 parallel LEDs. Each string of 14 is in one contiguous section. The forward voltage of the LEDs is ~2.5V, and the driver says 1.15A (I didn't actually measure it). So actually the voltage drop will be ~14x2.5V = 35V, the power drawn by each LED about 2.5 * 1.15 / 14 = 205mW. There are 14x14=196 LEDs total so 196x0.205 = 40.25 watts total. That's all assuming the power supply is delivering what it claims on the can.

A bunch of the individual LEDs had failed - about 10 on one side and 20 on the other. When 14 LEDs in a segment fail, then the panel doesn't light at all.

I suspect the failure mode is either low quality or over-driven LEDs. Once one fails in a 14-segment string, then the remaining 13 will split the increased current/power. Then they probably pop one by one, until all in a segment are bust and the panel goes dark. That could be the cause of the 1 Hz flashing death rattle. I didn't have the time to muck about further with this, but my current theory is that low temperature spell recently caused some drivers to deliver too much current. I could be wrong.

I've just ordered replacement branded Philips panels from CP Lighting item # 911401899780 at £30 a go - about 40% more than the previous supplier. Hopefully Philips test them better, time will tell.