My Jaguar combination boiler has suddenly developed a fault in that it doesn't turn off. Regardless of the position of the timer switch and having set it to summer mode, the central heating is on constantly and I have to turn the boiler off at the mains to prevent it from coming on. The overheating sensors do not appear to be cutting in and forcing the boiler to shutdown. Could you please advise the most likely source of the problem, before I contact a local maintenance company.