The two most common faults are wither grease\dirt on the glass or a faulty sensor.
If you have already tried cleaning the control panel surface with glass cleaner and confirm its free of water and dirt, then it will be the touch sensor.
Looking online your looking at around £130 in parts from suppliers here, however you will need to strip the unit down to confirm the part number.
If you would like me to recommend someone to come out to replace this part, please advise your postcode.
Yes, the part is relatively easy to replace, it is secured in place by a bracket which you unscrew, you disconnect the two ribbon cables, plug the new one in and secure it back in place. Its about 30 minutes work, but I would definitely strip the unit down first, get the board out and before ordering the replacement make sure the part numbers match up, the parts companies descriptions are not always 100% so worth calling them up and checking.
Appreciate if you can rate the service received, so I may close the case.