This is the rarest book I have ever come across. It's a book from my childhood that I rediscovered in the loft and have since read and re-read.
Mumbles are a kind of folklore creature, created specifically for this book. It follows four children, siblings and cousins, who are on holiday in Wales, near the real-life place of Mumbles Hill.
It's a good story with fair illustrations. Nostalgia is relevant here.