Ooo... this is a difficult one!
I think my Granny Weatherwax would have to be Linda Hunt (http://uk.imdb.com/name/nm0001373/
) as she is phenominal as Hetty in NCIS:LA. She has the correct amount of wit and intelligent about her, but god help anyone who crosses her! My only problem is the obvious one of her height. I always pictured Granny as a rather big and forboding character.
As for Nanny Ogg - thats the one that would clinch it for me - she is my favourite character so it'd have to be perfect. Although I have no idea who i'd have! I like the idea of Pam Ferris, but is she crude enough?
A bit of a random one, but I'd love to see Jennie McAlpine (Fiz from Coronation Street) play Agnes Nitt... I think she'd be perfect!