Thanks @Joseph-TP In Extender mode, I used the WPS method (pressing the button on PlusNet router, then this device), which meant that it auto-named the new SSID the same as the other and appended...

Just bought this to provide WiFi coverage at the back of my house & garden. The existing PlusNet (ISP) router is used for wired AND wireless LANs, with some extra switches located around the house...