Next up weekend after next, and hoping to do this:

http://www.brunel-museum.org.uk/tours.asp

even though I've already done the East London line.  However, at the time of doing it we noticed the Brunel Museum but didn't have the time to nip out and visit it, it being late on a murky Sunday afternoon.  The point of the project is finding all these random corners and places to see, so very keen to go back and do a tunnel tour. 

I have yet to decide on a time or whether to go on the Saturday or Sunday, so if other avid readers out there  wish to join me to marvel at the first tunnel under  the Thames, I post a poll to decide time and date::