Right, although I have an outstanding issue with Buddypress - it seems there's also a theme issue with Network as the Buddypress Defaul theme does not.
For the site http://retrofitdiaries.org/ the default (logged out) state is being displayed to all as being logged in (ie the label says LogOut).
When that label is clicked nothing happens (ie the login info doesn't appear).
This does work in a sub blog (temporarily) using the BP default theme: http://hounslowpassivhausretrofit.retrofitdiaries.org/
Maybe this is a theme or buddypress issue - BP 1.5.1 is a complete mess - but at least if this is a Template issue some gradual progress can be made...