Extensive testing has revealed occasional flickering.

I think the specs for the control chip are for 2A max, if I read it correctly, so understandably 2.8A is outside its comfort range. Switching power supplies tend to ripple in these situations so, I guess, hence the flickering.

I have ordered some R082 and some R100 resistors to replace the R050. This should put the power supply within its comfort zone and make the light work with any battery.