Permalinks settings won't save

I have just created a new install on a subdomain, and am trying to set the permalink structure before I begin development. I intend to develop the site, then point a domain name to it. When I try to save the permalnk structure to http://sub.domain.com/2011/09/20/sample-post/ and click save, the radio button reverts back to the default. When I then try to navigate round the admin I’m unable to without logging out. All menu links are preceded by http://sub.domain.com/wp-admin/options-permalink.php/ – so clicking

‘dashboard’ takes me to “http://sub.domain.com/wp-admin/options-permalink.php/index.php”

‘pages’ takes me to “http://sub.domain.com/wp-admin/options-permalink.php/edit.php?post_type=page”

Permalinks are set up on the main domain without issues. Any clues anyone?

Thanks