There's a wide sub surface walkway to just in front of kings cross
station from the overbridge at the former station, and its currently
disused.

Isn't that the tunnel which will be the main passageway from the
northern ticket hall to the Piccadilly and Victoria Line platforms.
See the sections near the "5" marks he

http://www.perry.co.uk/images/kx-com...with-lifts.jpg
No. Its the one just below the surface, its not a tube, its
rectangular. Here are some pictures -

http://www.abandonedstations.org.uk/...ngsCross51.jpg

http://www.abandonedstations.org.uk/...ngsCross52.jpg

this is where its sealed off:

http://www.abandonedstations.org.uk/...ngsCross55.jpg