235C: Hippo’s Birthday

I am looking for a book from my childhood which I remember very well. I am from the UK. The book must be more than 20 years old. It is about a hippopotamus, referred to as “Hippo” in the text. The Hippo asks his dad (also a Hippo) if it is his birthday every day. “Is it my birthday today?” and the dad hippo would reply, “No, not today, Hippo.” This goes on for some time as his birthday approaches. It is eventually his birthday and when he asks a final time, I think the dad hippo says, “Yes, Hippo. Today is your birthday. I even remember the dad hippo hiding the birthday cake on top of a wardrobe.