Sorry for the delay Kevin. Been off for a few days, at a conference on the British Bronze Age(!) and helping out excavating at a local mill where a mysterious floor has been discovered underneath the existing 19th century mill.
Anyway, that's pretty interesting. I would certainly not have guessed that was a crystalline cast of siphuncle from an ammonite. I knew it wasn't a belemnite, but beyond that I would have been stumped. How unusual!