My ferroli optimax HE25S does not shut down?

I have an intermittant problem with my ferroli optimax HE25S. Sometimes the CH side does not shut down when the timer says it should have. The HW is fine and has been OK all through summer. This prolem happens about 1 in 4 times ie I go to check it before we go to bed to find that it is still running even though the external controls are all saying no heat is being demanded. It means that I can't leave it on timer and go to work or to bed.