It's like the first image 🙂
Do you have any other sockets, and is anything else plugged into those sockets?
All extensions should be wired from the back of that bottom section that you have removed. But some old phone installations had extensions joined in before the master socket, which would not be isolated when you are using the test socket.
No we haven't got any other sockets so nothing else is plugged in anywhere. It is a fairly new socket I presume as the house is only 3 years old
If you have tried a phone directly into the test socket, and there is no dial tone, then it has to be an external fault. Why it happened after a power cut, is anybody`s guess.
You will just have to wait until Openreach visit.
Thursday afternoon is within the three working days for Openreach to deal with a fault. Its probably a short circuit somewhere.
Report the fault on a mobile by calling 0800 800 151.
Its also best to check at the test socket, to prove the problem is not on your internal wiring.