BuddyPress/WPMU SIte Options save to "Page Not Found"

Trying to save a change to the Site Options on my WPMU 2.9.2/BP 1.2.3 has suddenly started re-directing me to a Page Not Found message on a page with the default theme outside the Admin area – though this is the url displayed http://biguyz.com/wp-admin/wpmu-edit.php?action=siteoptions

It was working fine earlier today, and I’ve run a MySQL check on the database, and all seems well. Any thoughts on how I can fix this would be welcome!