That guide does not have any information that answers my question. We are using an external captive portal page, so cannot customise the internal login/logout pages. We just want to disable the login success page completely
The image you posted looks like the default Cisco Web Auth Successful page, which is internal. When external web page authentication is successful it should redirect the user to the configured redirect URL or the page the client tried to get to.
Could you verify the address that your redirect URL is configured to and post your custom external server captive portal code?
If you haven't already there is this document on configuring external web auth as well:
Ensure that the Scripting option is blocked/unblocked on the client browser as the customized web page on the WLC is basically an HTML script. On IE 6.0, this is disabled by default for security purposes.
The Pop Up blocker needs to be enabled on the browser if you have to block any Pop Up messages for the user.