There was a great story on the Today programme yesterday morning about secret codes. There’s a more detailed explanation of how the wartime diary was decoded here. I can’t resist a code, so if you fancy it, I present to you “The Blackheath Code”:
BHSSIBT KHEHNAU AAARDNY CTVEEEO LECADEH
Have a look at this,
Then answer question 4, and you should have no trouble decoding The Blackheath Code. You’ll find the keyword if you stare at the headline to this post. The answer will be published in a few days.
Further reading, and highly recommended (although entirely unrelated to Blackheath) “The Code Book” by Simon Singh.