Thanks for your reply.
From what you've said, I would imagine that it's the latter as the master socket in the kitchen was just a bog standard extension box when I moved in. I had a kitchen extension built and the builder updated sockets at that point so although I can't remember it happening (probably because I had more things to worry about at the time with the building work) I think that's when the NTE5 master socket was put in.
Prior to that, the other socket was added by someone I foolishly got out of the Yellow Pages thinking he would be running an extension from the kitchen into my office. He realised that that wasn't going to be easy (almost impossible I'd say) which is why he went outside to the "white box" and pushed through the wall next to that into my office to create a new extension.
So as you suggest it would seem likely that the junction box splits the incoming pair into two which, as you point out, isn't at all ideal.
I just hope that the OR engineer can sort all this out. I'd even be "happy" to lose the kitchen extension if I had to. I just don't want him attempting run a data extension from the kitchen to my office. It would be unsightly, hellishly difficult to fit and my wife would probably divorce me.