Cadmium, at 12, is the oldest of the Casson children. Next comes her adopted sister Saffron, who’s 8, followed by Indigo who’s 6, and Permanent Rose, born as the story begins. Bill, their stylish father, works as an artist out of a London studio while Eve, their scatty mother, looks after the family and does some painting to bring in money.

Caddy grows up quickly as her family focuses on her dangerously ill baby sister while her friends start to pull apart while trying to keep together. Eve goes into hospital unexpectedly and Bill comes home to look after the children while Rose, their new sister, struggles to stay alive. At the same time, Caddy’s friends are starting to take different directions. Ruby has been offered a scholarship to a private school, but is terrified of losing the support of the quartet of friends. Beth is concerned that the pony her parents bought is costing too much and that she is eating too much food by being too tall. Alison’s estate agent parents have been planning to move to Australia for a while but can’t sell their house. And Caddy, Saffy and Indy do what the Casson family do best: tumble cheerfully, chaotically, and with a quite understated love from one minor crisis to the next.

Caddy worries about her mother and the new baby while Saffy and Indy are insouciant and more than a little happy to have their father’s attention for once and to pass on the news they’re not supposed to have overheard about how fragile their little sister is. At the same time, Caddy’s friends are going through their own mini-crises.

Since this book is a prequel I trust I’m giving nothing away when I reveal that she hasn’t. But the point is that this is the day when Caddy’s bravery really shines through. In deference to her spider-wrangling abilities, her friends have affectionately bestowed on her the title “Bravest of the Brave”. But it’s a title which is really earnt this day in three quite different ways: as a public heroine, Caddy saves Ruby from going under a lorry; more privately, but just as bravely, she forces Beth to confront the reality of her size vis-a-vis her pony; and she finally musters her own courage to visit the hospital where she fears her baby sister has died.

If you read any of the other Casson Family stories, you get a feel for the warmth of the family: chaotic, funny, and affectionate. That’s definitely present here, albeit with the younger versions of the children. But here Caddy’s openhearted affection extends also to her circle of friends. They try to help each other, but each has secrets they find it hard to share. It’s not necessarily Caddy who sorts them all out, but she is at least a catalyst for change, even though it means breaking up a quartet which has been together since they first met at school. I’m glad, too, that the grown-ups in the background are at least supportive and real characters, not 2-dimensional charlatans nor buffoons for the youngsters’ wit to play off.

Tim Golden is a computer programmer living in London.  He is also the editor of the Good-to-Read website.