The Coronation Quest — Cecilia Peartree

'The Coronation Quest' is the third in the 'Adventurous Quest' series of historical mystery novels. It is set in 1953 in Britain, mostly in London at the time when everyone was preparing for the Coronation.
On a stormy night, Clemency Quest picks up a hitch-hiker on the Norfolk coast road. She doesn't realise at the time that this simple act of good-will is just the start of a sequence of events that will drag her brother, Oliver, and his fiancée, Flora, back into the complicated world of espionage and conspiracy that they have fought so hard to leave behind. As the Coronation approaches and Oliver and Flora prepare for their wedding, they are never quite sure which side of the law they are on.