Reopening a plastered wall to add optic cable

[jpguichard]

I got my porch plastered by my builder two years ago after the installation of BT optic fiber. This was to hide the hanging cables. Now I would like to have Community Fiber running alongside BT to test and switch to whoever is more reliable later.
See the picture with the cable arriving at the top then going to the bottom box then going to the right.
What is the cleanest solution to open and add new cable for community fiber while not cutting existing cables?

 

[jj4091]

Are you saying the exsisting fibre cable runs from the ceiling down to the box shown then turns right to somewhere else. Is it clipped to the wall under the plaster board or hanging loosely.Is there any spare cable behind the plasterboard. Is it connected to a BT box at the bottom. How much room is there behing the plaster board. My initial thought would be the easiest and neatest solution would be to put it in some 10mm trunking down the corner of the recess oposite the window. No need to disturb anything then.