Postcards from a stranger - Sally Stewart

Returning home to Little Fairford, Oliver Hatton finds village life reassuringly unchanged - except for the transformation of Jane Kingsley, once a gauche, unhappy teenager.
Her uncle's priceless collection of ancient scientific instruments attracts study from a visiting researcher, Jules Legrand, but after a burglary and the death of Jane's uncle, Oliver distrusts both Jane and Jules, particularly when the instruments left behind are found to be fakes.
Torn between old and new loyalties, Jane goes in search of the missing collection, with Oliver at her heels, following clues that lead her across France to danger and despair...