It seems to be the week for data leaks at the moment. BG have stressed that the leak was not related to a hack which is reassuring and they will be in contact with the customers affected. The volume involved is proportionately really low.
From the article it seems to be related to situations where people tend to use the same password for multiple accounts or websites. There is a good article around password protection here
as more information comes out about this story, we will try and keep you updated.