OK I though, maybe he's really tired, so I reload and hit the velvet myself and go see him the next night when he's full of life... Er full of death.. and start again.
"Follow me" he says, before walking to the upper balcony and stopping, never to move again.
I've watched how the quest should work on a youtube video and noticed something wrong right away. He doesn't talk about the quest as he begins to walk, instead he repeats the standard dialogue ("I saw more than enough political maneuvering in my time blah blah") and instead of walking down the stairs to the main room, he walks right across to the upper corridor.
I'm running a few mods, so I've gone through the process of disabling them with no success. I've used TES Edit to see if any mod changed ANY value in Dawnguard and if so, tried to see what, in case it left a mark in my save file. I can't see any references to him, the quest, or even the location in anything other than the unofficial patch (Which I disabled).
After quite a bit of searching, the only problems others seemed to have with him was him not recognizing the player was a vampire when not in lord form, which isn't a problem for me.
Has anyone any ideas?