I would definitely be in favor of a replacement. It would still be '24 sussex' but could use much more modern design, customized for security concerns and energy efficient as well as impressive. And it could use 'salvaged materials' from the original to ensure historical continuity. Perhaps some of the interior design (the good parts)  that diplomats might recognize in the meeting halls. 

Much cheaper and more prudent than a 'refit' of crumbling and outdated facilities.