All ceramic hobs are controlled by effect regulators. When the ring is full on all the time the regulator is defect and needs changing. The regulators have a number on them which is what you need to order new ones off the net. The last two numbers tell if they run on 240 or 415V. To change, turn off the power then remove leads one by one putting them on to the new reg so they dont get mixed up.. Easy peasy..
Steve.
Steve Hand, August 2007