The Other Blond at Number Four

Dudley shouldn't be surprised that it's Malfoy, but he can't help the bewilderment that overwhelms him when he sees that mean little face screwed up in pleasure, staring up at his cousin with starry eyed adoration.

or,
Dudley finds out about a terrifying blonde freak straight from the dark side (!) shacking up with his cousin right here in his home. Naturally, he goes snooping...