The Pit-Prop Syndicate (Action-Adventure Thriller). Freeman Wills Crofts

Seymour Merriman would not have even imagined that a little bit of help from strangers would catapult him into the world of crime syndicate, spanning two continents. He had only followed Lorry Number 3 in the hopes of getting some petrol for his bike. But that ended up with him noticing that the number plates had been changed! Why were the number plates changed? Who were these people whom Seymour had inadvertently met?