Potterton Profile 50e heats radiators continuously?

I have a Potterton Profile 50e, approx 10 years old never serviced, it started to take about 8 or 9 attempts to fire up, can hear fan going then clicking sound of pilot trying to fire up.Once fired up,works fine,heating & hot water.Now the heating will not switch off even with thermostat off, have to swich off power to timer to stop it altogether.Any idea's??