Unsolved Mysteries: The Salisbury St. Ann's Gate Cipher

I recently attended a talk on the cracking of the Enigma code, in which we were able to get hands on with one of the machines as well as learn more about its history from a presentation given by Dr Mark Baldwin. It got me thinking about the many unsolved cipher mysteries out there and I was amazed to learn that I have a coded mystery right on my doorstep here in Salisbury. I don't think I'm clever enough to solve it though. The story goes, that in the short period after the Second World War, an army truck was driving through the Cathedral Close here in Salisbury where it struck the stone archway of St. Ann's Gate, one of the entrances into the Close. There's actually a picture of the damage at the time. When the damaged stone underneath the window had to be removed, it apparently revealed a hidden cavity within the arch. Tucked inside was a weathered piece of parchment comprising of a page of indecipherable letters. I could only find part of the Salisbury cipher code o...