Susie Day: Pea's Book of Best Friends [2012] paperback

When Pea Llewellyn's dizzy but dazzling single mum becomes Marina Cove, author of the bestselling "Mermaid Girls" books, everything changes. It's time to leave their tiny flat in Tenby for a proper house in London, and a whole new life. Pea likes the red front door, and the attic bedroom all to herself. She even likes her hideous new school uniform, in a masochistic Malory Towers sort of way. But there's an empty chair beside her in every lesson, and no one seems to want to fill it. In the absence of volunteers, Pea is going to have to acquire herself a best friend...