It seems to unfortunately. e.Selection is contains no objects.
It's a rare case that I need to use it. I'll prompt the user that the other entity needs to be modified and not the one selected.
The selected objects are in the AddedObjects property, not the SelectionSet property.
Read the documentation, and search here for examples of its use.
I 've ended up using the EnteringQuiescentState event to do this.
I couldn't understand the errors the SelectionAdded event was generating or why it was firing so many times.
Thanks again and sorry for any trouble there.