Creda Electric Fire will not work?

Creda Model 81802 Electric Fire will not bring on the heat elements - Switch on and the bulbs light - been all over it with a an electrical test pen and get power everywhere at all contact points - glass heat elements are working - tested elsewhere - but still no heat when reassembled - could it be the control rheostat that controls the amount of heat? There is also a small black plastic sealed cable at the exterior back connected to the circuit board by two wires - what does that do??