When trying to create a custom tab/app on facebook, there is no 'create app' button present on my page. Have I missed a step in the setup or is there a reason behind this? Any help will be greatly appreciated.
Are you trying to develop your own app, or adding an app that has already been written?
If you go to an existing app that is intended to be added to a page tab, it will usually have a button/link to add the app to your page. If you click on this a dialog should pop up and let you choose the page, from the pages where you are an admin.
If you have written your own app, or the app does not have an Add to Page link, these instructions on the Facebook Developers site show how this link can be added to your app or entered manually. If you want to enter it manually you can copy and paste the following URL to your address bar: https://www.facebook.com/dialog/pagetab?app_id=APP_ID&next=https://www.facebook.com/connect/login_success.html (Change APP_ID to the app id you want to add. The app id is a number which can be found in your app settings, or in the URL for the tab if it has been added to another page.)