LOL bit like my face then

While we're chatting I started to explore the blend shapes idea but the model that I was working on was one great big mesh (Knobby - you might have seen hime he's my avatar). I found that when I separated the head into several different versions to create different shapes it didn't work...

In the end I had to duplicate the WHOLE mesh and create blend shapes, kinda make the file size LARGE... Am I doing it all wrong or is there a way of connecting a severed duplicate head?