I raised the seat height to max and had a look under the front of the seat, removed the plastic guard, and there it was!!! a Brown 2-pin plug just needed clicking back in properly

Tested the seat, and it was all toasty again

I don't think it was the Power Supply to the heater but rather a sensor of some kind as the wiring was thin gauge.
Has anyone noticed that the seat will not heat up unless you are sitting on it? I wondered if this brown plug was part of the controls for it?