Amusing. Believable OC and Mr. Dursley is IC.
Grammar etc:
No big problems in here. Certainly nothing that got in the way of the story.
The N in "Number Four, Privet Drive" is usually capitalized.
"purple faced" should be "purple-faced."
"Never mind" is two words. "Altogether" is one word in this case. "Self-appointed" is a hyphenated word.