Ferolli SYS 10-23 Boiler runs continuously?

Hi there.
I have a Ferolli SYS 10-23 gas boiler. The boiler is run from a Grasslin QE2 timer switch. Recently the boiler will run on whenever both the CH and Hot Water have been ON for more than about 30mins. This is in spite of the timer commanding the boiler OFF! The CH remains OFF (ie motorized valve stays closed) but the boiler runs and runs heating the water.
I've tried replacing the timer control and this has not solved the problem. I suspect a fault within the boiler itself. Can anyone help??