Remove the wires from 1,3,4 and6 in the extension socket, remove the wire from 3 in the master. Make sure the 2 wires in 2&5 are pressed fully into the IDC slot. The extension will only work when the faceplate is re-connected on the master.
Do you know the cable route from the main socket to the extension? Could it be damaged?
Is that the only extension socket you have?
I don't know the route the cable takes, the house is a new build (6 years old), I've never touched any cables so I wouldn't have thought they could be damaged.
There's a extension socket in the living room which works, the socket in my son's bedroom which doesn't work and there's also a socket in my room upstairs but I can't check it as it's behind the wardrobe. I've never used either upstairs sockets.
it's quite probable that the upstairs sockets are connected in series so the cable from master goes to one socket which is then connected to second which is connected to third so could be first socket is one behind wardrobe
You need to check the wiring in all of the sockets, I suspect one of the wires has become disconnected in one of them which means the sockets further along the daisy chain won't work. If the wires aren't fully pushed in to IDC slots it is quite common for one to pop out particularly when there are 2 wires in the slot.