The Matilda Effect by Ellie Irving

Matilda loves science and inventing, when she doesn't win the school science fair, because the judges didn't believe she'd come up with her entry on her own she is devasted.

When Matilda learns her grandma was an astrophysysist who was also overlooked when her boss, Professor Smocks, stole her discovery for himself she is astonished! So begins a race against time - and against Matilda's strict mum and dad! - on a journey through Paris, Hamburg and Stockholm, and on which they encounter a famous film star, a circus, and a wanted diamond thief...