I visited The National Museum of Computing at Bletchley Park last November with a small group from my University. Tony Sale, who lead the team that rebuilt Colossus, gave a talk about how the Tunny machine worked as well as Colossus and the rebuilding process. We also got a demo of Colossus in action, it was really pretty incredible.

If you live in the UK you should totally visit, they've got some really unique hardware running and everyone is so enthusiastic and wants to teach the visitors about computing.

http://www.tnmoc.org