If you are near Twickenham - I found a shop called Cutters in St Margarets who are selling Green and Blacks 100g large bars of chocolate at buy one get two free. Stock is dated end Jan 2010.
Sorry if you are not in this area but it is the cheapest price I have ever seen. They have 1000's of them various flavour but mostly milk chocolate.
Update - went back today they are called Cutters Ltd Not Costcutters.
They also have BOOST 12 DUO BARS £2