Why does boiler burner ignite outside programme times ?

I have a Potterton FlamingoII RS 40 the burner has begun to ignite idependently when no heating or hot water is required. When the boiler thermostat is turned to O(off) the burner turns off and returns to pilot light,if i then manually overide the programme settings using the instant heat/water button the burner ignites but when the programme button is pressed to cancel the request the burner stays on and the only way to turn off the burner is to turn the boiler thermostat to O (off)