He put the receiver down and, while stroking his moustache, Mr Dursely thought.
I'm being stupid
I'm really stuck on the line in bold.
So I guess I can translate まさか as "really". I think it intensifies the rest of the sentence. Is this right?
I think なんて in this case is "how..." so まさか、なんて愚かなんだ would be "Really! How stupid I am!"
So, if what I've got so far isn't gibberish, I don't know how to fit in the 自分は part. Literally I suppose it would be "Really! As for myself, how stupid I am!".
But, is 自分は really necessary? Why? How should I think about it?