Dangerous Diamonds by Barbara Mitchellhill
Published by Andersen Press.

When Dad goes missing, twins Harry and Charlie scour Edinburgh to find him. But why are others determined to stop them? And how is a strange wooden box linked to his disappearance? The twins soon find they are in terrible danger but push themselves to the limits in their attempt to outwit those who are holding their father.
About Barbara Mitchellhill
Barbara Mitchellhill is lucky enough to live in a village in Staffordshire in a large sunny house with a big garden - nothing like the tiny, dark house where she was brought up in Rochdale. She and enjoys taking her dog Ella for walks over the fields. It's a great way to think up stories. When she’s not writing, she likes to read or go to the theatre or listen to music or dig in the garden or see friends and family. There never seems to be enough time to do everything she would like to do!