@jonny Go to the settings of your wifi router and switch off SSID broadcast. Make the name of the router something easy to remember & spell. it now won't show in the network list on your phone. use "add wifi" (or whatever it's called on your phone / computer) and manually type in the wifi name & password. done
This isn't some kind of wifi magic. Someone really looking for the wifi will still find it but it's no longer visible on the list of available networks