Hugely interested to read of this trip up the tunnel and see the photos, as I did the same exploration in the mid 1950's in a home made wooden boat with a school mate. The condition of the tunnel looks just as I remember it from more than 50 years ago. I don't think we told anyone where we were going so it could have ended up as another great missing children mystery. My mate lived just by Coldharbour bridge, from where we carried his "boat" to the tunnel entrance. We were also limited by the cave in.