My first thought is that the box is feeding two extension sockets in the flat, and the master socket is elsewhere, possibly with those for the other flats in the basement or similar.

Why is it feeding two Bob ? One cable in (almost) and one out.The corrosion seen on the IDC terminals should not be an issue, as these have been bypassed by crimping through. The set of 'square' crimps do look a little manky TBF.

The second picture is where I got that from, though it looks a total mess.

There appears to be two connections into Pin 3 on the lower bank. The upper bank also seems to have connections, though maybe they have been cut off.

It's rather hard even to see where all of the used wires exit to the flat .

Yes it does seem to be an awful mess! I took some more photos with the wires separated a bit to clear things up. Also took some outside the flat. Bob, the only cable that exits to the flat is the white cable to the right of the box which goes all the way to the living room to the NTE5/MK2 box.

Inside flat:

http://oi40.tinypic.com/33ynnyp.jpg
http://oi40.tinypic.com/ajv2fo.jpg
http://oi40.tinypic.com/27yxrtx.jpg

Outside flat:

http://oi44.tinypic.com/wsn1jl.jpg
http://oi41.tinypic.com/347baxs.jpg

The line seems to lead down the building to this box:

http://oi39.tinypic.com/2a7fxtt.jpg


Do you think the line from the box downstairs to the flat could be replaced with one single new cable straight to the socket in the living room? It doesn't seem like an easy task haha

OK, I'll agree then, that all looks pants. If it's BT Retail you are with, see if they cannot raise a 'Boost' visit. Then show the engineer what you have shown us.

It looks to me like the cable goes over the top of that locked box, rather than into it. Whatever is in the conduits (possibly fire alarm cabling) is what goes into the box, unless the phone enters via the top of course.