I just ran:
$ heroku fork -a oldapp newclonedapp
and it worked fine and runs etc. Now I want to pull the code down to work on it [I realize
You're not doing anything wrong, it's a known issue of fork that it doesn't clone the source app repository. Until the issue is resolved, you'll need to manually clone the source repository. Here's how I'd do it:
$ git clone git@heroku.com:oldapp.git -o old newclonedapp
$ cd newclonedapp
$ heroku git:remote -a newclonedapp
$ git push heroku master
Basically, you clone the original repo to a new app directory, setup the heroku
git remote and push to it to populate the new app repo.