ok i understand atm this option is being controlled through the 'bbshowhidden' cookie - am I correct?

The problem with this is however if someone then logs on from another machine - or clears their cookies the preference is lost and must be reset by creating the cookie again.

Would it not make more sense to store this as one of the users prefrences in the mySQL table - that way no matter where the user logged on from it would remember their last choice and the forums could react accordingly?