I have an Sime Friendly combination boiler in my house, but no instructions. There is a switch that allows me to choose between constant and timed, but I can’t figure out how to tell it when to switch on and off. There is a dial which appears to have 24 hours indicated on it, but there is only one arrow. I can see that this will allow me to specify either an on or an off time, but not both. Anybody know how to use this feature of the boiler?