Just to clarify before I blow something up:
When connecting the external power supply to the heated bed do you leave the ribbon cable connected to the main board, or do you have to disconnect it ?
I've answered my own question here by finding this excellent video describing the heated bed in great detail:
https://www.youtube.com/watch?v=6ZlT1WpGr6o
Essentially there are two independent heater elements in the bed !