I think I've finally got it though it doesn't make sense - look at the groups and you'll see.

I think the only way of each blogger having his own space where they don't or can't write in their neighbours is to make a group for that one blogger and add the blogger to the group and give permissions accordingly for that group with only one blogger in it. If someone wants to be able to edit that bloggers posts then they get added to that group. It's nonsense but it works. Why a group? I don't know - I've been testing it all day and this is the only way I can do it.

I also made a group for the user dublinstreams but I can't find the user now ..