DXP55 I like your thinking there. I think bringing the cable in to the house and terminating to a master box sooner rather than later is probably the route I'll take.
Right now, I'm thinking of two options:
1. Keep the cable coming in the way it does, but run a new cable from it straight down, into the basement, and then come back up in the top right (as you look at the diagram) corner of the sitting room, terminating in the socket.
2. Follow the outside cable up the wall a bit, add a junction to a new cable (is that even possible/advisable?) and drop it down the outside of the house straight down to come in to the sitting room where the arrow is.
Bonus third option:
3. Option 2, but take the cable down, run it at low level to the bottom left corner of the sitting room, and in there. Low level as I'd rather not have any more cables visible across the front of the house.
I'm comfortable changing the current set up, just need to decide the best option now.