I am trying to set-up a user to only edit one page within an intranet and not
edit/publish any other page.

I originally tried to set-up the custom permissions but no joy, so then I setup
custom permissions with all options ticked, the user is able to edit the page
but not publish it.

The user has read access to the Intranet. If give the user access at the site
level, he/she can edit all the pages and publish them.

I have the following setup:
MOSS 12.0.0.6219
Publishing Infrastructure features are activated.