Ideal Mexico boiler does not switch off?

I have an 18 month old Ideal Mexico HE18 floorstanding
boiler. When its timed period comes to an end the
boiler does not switch off and of course the pump continues to run. The control panel shows a
'C' and the burner light flashes. Pressing reset does not do any good and the only way to turn it off is to
cut the power at the mains switch.
Any ideas?