Fixed this by deleting wp-admin and wp-includes and then copying both folders back from the original wp archive. Thanks to the guys on the WP irc channel for the help.
OK I have managed to dig a little deeper into the issue. There are errors in wp-admin/widgets.php as follows:
Sortable is no defined (line 329)
Draggables is not defined (lines 165)
Anyone know how to fix this?
Thanks
