Worth a visit
Me and TOH visited Mail Rail and the Postal Museum last year. The journey on the former mail bag train was pretty good, while somewhat cramped for a lanky bloke like me! It was relatively short, compared to the full network, but still interesting to see with interactive presentations that run during the journey. The volunteers there have done an excellent job of getting what is, in essence, a former freight only line working for passengers from the general public to ride on safely.
This tunnel walk is a great addition so will have to book up for it. We've done similar things in the past including the tunnel walk from Rotherhithe to Wapping and back through the Brunel Tunnel (excellent!), and also the visit to Aldwych station (also worthwhile).
The postal museum was pretty good too. The only thing I would say is many of the interactive displays in the Mail Rail site were broken when we visited. I'd like to think they've sorted these out now, and if they haven't the really ought to as they looked pretty interesting (I think one was a demonstration of signalling operations on the line).