This is book #2 in the Mystery Club thriller series. I read them long ago, after buying a few of them in a London bookshop. I liked them so much that I hastened to buy the rest of the series after we got home. I'm enjoying the re-read. A series for younger, mid-grade, kids is called "The Mystery Kids". It also involves thrilling adventures of three kids in London. One of those kids, Holly, is now a teenager who lives in a small town in Yorkshire. Together with Tracy and Belinda they have formed The Mystery Club.
In this book, Holly is out on her bicycle when she witnesses a car go shooting by, and crash over a hill. Climbing down, she finds no one in the car, but she had seen a man leering out the window at her as he sped by. So begins a dangerous mystery of a man who will stop at nothing to get what he wants, and this includes getting rid of the three girls who are meddling, trying to help his elderly father and secretary.