I don't know if you've read this Susan Cooper interview before; you might find it interesting:
http://www.lib.rochester.edu/camelot/intrvws/cooper.htm
Especially the bit where she says she always envisaged Will and Bran getting together when they grew up.

No, sorry. I just made that bit up. It's not actually interesting in that way.

And a random interesting fact: did you know that Chupa Chups wrappers were designed by Salvador Dali?