While we're on the subject of free museums, if you're into military aviation then head out to the RAF Museum which is about 5-10 minutes walk from Colindale station on the Northern Line of the Underground - well worth a look. :thumbsup:
I'm going to pull you up here Stat.
I LOVE the NHM, and a big part of what I love about it is that when I walk in it looks basically exactly the same as it did when I visited as a kid and if I visit in 20 years, it'll probably still look basically the same. It's not tired to me, it's comforting...