Hi Calvin
Thanks for all your help. We have managed to fix the problem.
We rolled back all of our servers and database and ensured that Oracle client was installed on all of the servers and then ran through the upgrade and everything worked fine. So the issue seemed to be that we stupidly uninstalled Oracle from our web servers to free up space and then did the upgrade and installing Oracle client after this didn't fix the problem so couldn't connect to the database again.
So the moral of the story don't try and free up space on your server by uninstalling Oracle DOH!