Old Wizard Whateley: Chris Cooper http://www.imdb.com/name/nm0177933/?ref_=fn_al_nm_1
Doctor Armitage: Anthony Hopkins http://www.imdb.com/name/nm0000164/?ref_=sr_1
my favourite actor. He could make the phonebook sound portentous.
Wilbur Whateley: Ezra Miller http://www.imdb.com/name/nm3009232/?ref_=tt_cl_t3
This kid can do spooky with the best of 'em. Perhaps his voice isn't deep enough though. Full CG head blended with a live action body for the tightly buttoned up Wilbur is doable (I used to work in movie VFX)
Lavinia: Sarah Polley http://www.imdb.com/name/nm0001631/?ref_=fn_al_nm_1