The temperature of the heat stored is controlled by a thermostat.
The amount of heat released is controlled by a mechanical flap.
It sounds like the thermostat is stuck shut.
Turn the power off, remove the outer case of the heater and turn the heat control. You should see/hear the stat open and close or use a meter to confirm the operation. If it doesnt operate then it probably needs replacing.
You can get the spare part here:
http://www.storageheater.co.uk/Models/Creda/79141C.htm
But at £50+ for the stat you may be better off with a complete new heater,.