Can anyone help, please? Have recently become owner of 12/13 years old Vokera Excell 80SP combi boiler, which is due its annual service. However, have so far not managed to decipher the markings on the small slider switch that lies within the 24 hour tappet clock. It is a tiny bit of plastic that can move into three positions, but I cannot make out what it is for. (I know!).