its not rocket science wiring an S plan, but you would be so surprised how many get it wrong, by reading your first post, I suspect that a tsome stage when someone has been changing one of the actuators they have mixed up the wiring, everyone else that has come along has put a new on on are a new valve and followed the same wiring, trust me very very few BG engineers would actually trace the wiring to ensure it was correct they will just change the actuator or valve, this is not BG bashing before anyone says that it is sadly the same for a very large amount of the industry, as you are now a qualified heating engineer I will explain in simple terms how your system works, you have two zone valves one for heating one for HW,the valve is a valve operated by a motor , when the valve opens , a physical mechanism closes the contacts on a micro switch, both valves have a separate micro switch, both micro switches have a permanent live into the grey wire (this must be permanent and not switched for reasons that have already been explained ) when the switch is closed by the physical mechanism then voltage is sent to the orange wire, this tells the pump and boiler to operate, when the demand for either HW or CH is turned off the valve closes and the micro switch on that valve is now in the open position so no demand from that valve to the boiler, when both valves are in the closed position then there is no demand for the boiler to fire, yes it is common for end switches to seize in the closed position but you have had the actuators changed so it is not that, at some stage someone has mixed up the wiring and everyone following on has just followed the same wiring
