I also had a look inside the neighbour's tunnel. It is roughly the same construction but the ceiling is quite a bit lower. Whilst it's drier, it's also quite badly corroded, all the way through the steel, in some places (see second picture).
The neighbour said that a "bunker hunter" had once pitched up and told him of rumours that the tunnels in this area all interlinked! The back of his has collasped under rockfall, so who knows! Man, people love a good underground tunnel rumour!
It also has the same wiring setup along the side of the sheeting, which the neighbour had always thought was some kind of comms wiring. In my mind this suggests that the wiring in both is original, and not a later post-war addition.
Finally, a night vision video that Dave edited from inside the tunnel (sorry gamers, no Wolfenstein to be seen) ! We still haven't started clearing yet, as I promised that I'd try not to start moving stuff around until certain people have had a look.