Resolved it, adding design time data helped...though weirdly with it working the design time data now looks wrong. I'm not 100% this is fully correct, or the best or simplest way of doing this...but it does seem to work so far. I've got a few CollectionViewSources, but the first and last are the only two relevant to this.
I needed the CollectionViewSource unitNameViewSource, so that I could set the ItemsSource on the ComboBox to something. If I didn't set it, or set it to {Binding} I got false values on the first record. The ComboBoxs are defined as:
The other thing I think was important was to bind to BusinessArea and not the BusinessAreaId column. Let EF faff with translating that! My code behind is then:
private void Page\_Loaded(object sender, RoutedEventArgs e)
{