I, Spy A Bletchley Park Mystery by Rhiannon Tracey

A book for nine to twelve yr olds. 
Robyn has been assigned to help with the carrier pigeons at Bletchley park. She’s twelve now but because her dad is a driver there, she has grown up helping him in the garages. Her new role means she has to sign the official secrets act & must not leave the park grounds. Bletchley is a busy place but Robyn is convinced something sinister is going on…& she & her friends plan to uncover exactly what it is