Quest for a Father — Cecilia Peartree

This is the 6th book in the 'Adventurous Quests' series of thriller novels set in the world of the 1950s, when people hadn't quite recovered from the war and were on the brink of being plunged into a cold war stand-off with Russia.
'Quest for a Father' takes Clemency and Andrew to Spain, at a time when recovery from a different kind of war was even slower, and there were secrets and dangers round every corner.