White Knight Condenser dryer will not heat up?

I have a White Knight condenser tumble dryer, which will not heat up. The drum rotates back and forth, stopping and changing direction, it just will not heat up either on the full heat or half heat setting. I have cleaned the condenser filter unit etc to no avail. I have also tried the reset button at the back, but this is already pushed in.
Suggestions as to what to try next please.