After working through this problem with support I now have it resolved. Here is what I did in the end:-
Installed a custom version of the United Kingdom (Apple) keyboard input language layout:-
https://dl.dropbox.com/u/43876768/United Kingdom (Apple).zip
(This ZIP includes the source code for the layout, ukapl.klc, and the setup, created using Microsoft Keyboard Layout Creator 1.4 based on the United Kingdom (Apple) from bootcamp windows drivers. Installing this avoids the need to install the bootcamp windows drivers pack.)
Note that, this is a tweaked version of the Apple defined layout to move # from Ctrl+Alt+3 to Ctrl+3 to avoid menu bar flicker.
Then back on the host, in Parallels Desktop -> Preferences -> Shortcuts -> (select VM) -> Profile: Windows
I added a mapping for Alt + 3 to Ctrl + 3.
I can now press Alt+3 (or Ctrl+3) in my windows guest and get a # !!
Last edited: Oct 10, 2012