Grumble. Okay, this is sorted/ There are amazing numbers of not-quite-right examples on the web.
Thanks, Tom, I now have the below, which works.
#+TITLE: Console Tic Tac Toe
#+SUBTITLE: A Literate Program in EMACS Org-Mode
#+AUTHOR: Charlie Martin
#+STARTUP: showall
#+BEGIN_SRC python :tangle yes :noweb yes
import sys
import os
def main(args):
<<initialize-the-game-board>>
if __name__ == "__main__":
main(sys.argv)
#+END_SRC
#+NAME: initialize-the-game-board
#+BEGIN_SRC python
board = [[-1 for x in range(3)] for y in range(3)]
#+END_SRC