It the Printed Circuit Board that controls the power.
It is located within the unit near the top slightly off centre.
It is rectangular and its base colour is usually, but not always, green. It has a number of electonic components soldered, attached and 'printed' on it.
Replacement boards cost about £70 to £80 pounds.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.