Is my frost stat faulty?

My frost stat (located in the garage) is kicking in and firing up my boiler even when the control is OFF. That's fair enough, I understand it needs to protect my boiler.

However, the boiler runs and runs and runs, never switching off. This is causing the water in the cylinder to be extremely hot, with fluctuating water temperatures coming out the shower (it can be OK for 5 mins, then suddenly have a blast of scalding water).

Is my frost stat faulty?