I loved this. I seriously felt as if I were listening in on two real people. You've done an excellent job characterizing Piers; the magical details are imaginative and make perfect sense; Dudley comes into his own beautifully.
Their language is just so exactly what I would imagine. I wanted to quote some favorite lines, but I realized as I looked back that the story is just so much of-a-piece; it all flows so well, each image and insight building on the last, that I can't really excerpt things very well. But I loved the several paragraphs beginning "Piers wasn't all that cracked," and this line: Piers scrubbed at his wet cheeks; it was just sweat, really, and the catkins making every inch of his face crawl with itches.
You really give us a sense of how difficult the magical world made life for Dudley, something canon, for various understandable reasons, doesn't really explore. Finally, I love the way the story fits so well into canon. The lost DH year from the Dursleys' pov makes for a fascinating story. A fine story, perfect for Beholder.