In the first one, you are confirming something you already believe. Perhpas you want to talk more about it with the person and you need to confirm that you have it in common.
You've seen the last Harry Potter movie, haven't you? You've eated at Banburry's, haven't you? You've been to London, haven't you?
In the second one, you've just learned a new piece of information. It can
Thanks guys. I had thought that we had to seperate the tag from the main clause in the second example into a new sentence. My grammar really needs revising!
I don't know if the grammar part of a language is more likely to be forgiven than the other parts of the language. Or maybe it's only me, I'm getting old, I know that!