Basil Jet wrote on 16 September 2013 00:20:18 ...

https://maps.google.co.uk/maps?hl=en....08 ,,0,-0.18

It seems to be simultaneously too far from the line of the tunnel and
too near to the mouth of the tunnel to be connected with it, and yet I
suspect it is connected to it, but what is it?

It certainly looks like a vent/intervention shaft for the road tunnel.

There is an emergency exit in the offside wall of each tunnel bore,
roughly at the mid-point of the tunnels, just south-west of where they
pass under Leamouth Road. (Look for the green signs in the tunnel on
Street View.) You would expect there to be an exit portal on the empty
patch of land above it, but there's nothing. So perhaps the emergency
exit doors in the tunnel lead to a path just below the surface which
ends at the mystery structure. Possibly there were problems obtaining
access to anywhere closer.
--
Richard J.
(to email me, swap 'uk' and 'yon' in address)