It's very interesting how much Harry Potter actors have parts in other shows. This one has David Tennent (Barty Crouch, Jr.), Rupert, and "Oscar-winning actor Jim Broadbent" (Professor Horace Slughorn).

To me, this feels like Harry and Hermione's response to the first time they heard of "The Tales of Beedle the Bard". As someone raised outside of Britain, I've never heard of Postman Pat but others are saying that this is a very common show and "everyone knows it".