Thanks for the replies. I'd already worked out that I was barking up the wrong tree. I was asked by a client to create a PHP script to disconnect a user. I'll probably use a script to change Expiration or Max-Session and get him to get the NAS to check back to Radius every five minutes.
It's a pity that you can't send a disconnect request directly to Radius for a particular user. Anyway, once again thanks for the advice. Regards, Sean - List info/subscribe/unsubscribe? See http://www.freeradius.org/list/users.html