The terminology used has been confusing throughout this post. I'm more inclined to trust pictures than words.
I think the only DIY compatible way forwards is to establish that the device with a 3 amp fuse in it is indeed an FCU, and establish what the cable is connecting it to the single socket.
If we could have
-a photo of the FCU which has the 3 amp fuse in it
-a photo of what we are being told is the single socket the other side of the room that is connected to said FCU
-with the power off, a photo of the wiring behind both of these
...then depending on what the photo's show there might be a relatively easy DIY way out of this.