Originally Posted by Stu @ M Developments

I was right, i just asked him for you and he knew off the top of his head.

He said the black one is the signal output wire and goes to pin 2 of your L8/P8 and the white ones are the heater power supply and thus need wiring to live and earth. These are NOT polarity conscious, so wire them how you like.

Hes a top man.