I would put into play a Nanny one who is a friend of Elizabeth's and help (for a time) raise Sean. A nanny would go unnoticed by the royals but could easily over hear everything. Even with the Kings children raised there could be other children in the castle for time to time. The King had brought a nanny to Portland for Diana (a hidden clue) Sebastien was great but even someone like him might not be around the general family when they are talking. Sean needs information on the happening in the royal families now more than ever. Who else could you see taking his place?