Thanks for that reply.
But I'm still lost. Once I put the button in my blog template I have no further control over it.
Do you mean that anyone who wants to tweet one of my blog posts from the homepage has to amend the reference themselves as they tweet it? If so that sort of defeats the purpose of the button. I was hoping I could tweak the button so that it offered the permalink to the post rather than the url of my blog home page eg "http://photopol.blogspot.com/2012/05/in-defence-of-twitter.html" instead of just "http://photopol.blogspot.com/",
I notice http://daithaic.blogspot.com has succeeded in doing this but when I nick his code it doesn't work. Perhaps it's because I'm on an old blogger template and it's just not geared up to it.