Thank you for all of the advice so far.
On closer inspection, the Revelation is definitely not in use - there is no power going into it.
I am however now unsure of where the line actually enters the property. I thought it entered through this cable at the bottom:
https://ibb.co/KKKhbwfDirectly in front of this cable are these boxes, underneath the Revelation:
https://ibb.co/8dvmGzxPerhaps they are not in use. Equally, this external cable might be nothing to do with the line - maybe TV aerial or something electrical. It definitely comes from underground though.
The line then seems to just appear at the top of an external wall, run over the roof and go into this box:
https://ibb.co/c8QZYPWThen it goes round the edge of the house and into the first master socket.
So I do not actually know for certain where the line enters the property, frustratingly. There are no other entry points for cables anywhere else on the outside of the house, with the exception of that shown in the first image. Any ideas?
@Black Sheep - no, I have not had a DSL service while living there. The previous occupants might have though. Is there any way of telling?