Stones of Treason — Peter Watson

An art historian gets caught up in a deadly international standoff in this “inventive” political thriller (The Guardian).

What would happen if Greek extremists blackmailed Britain into returning ancient sculptures? And what if that blackmail involved not only Queen Elizabeth II, but also dark secrets about her uncle, the Duke of Windsor?
 
When Edward Andover, Surveyor of the Queen's Pictures, receives a painting by Raphael from an anonymous collective known only as the “Apollo Brigade,” he's nonplussed. But the ominous delivery is soon followed by two more. All three paintings, it turns out, were looted by the Nazis during World War II. Soon, Andover finds himself racing to prevent the public exposure of secrets from the royal family's past, should they not meet the group's demand to return of the Elgin Marbles from the Parthenon, on display at the British Museum. What's more, his very life may be at stake as the Apollo Brigade makes its violent intentions known.
 
Set over the course of five explosive weeks, Stones of Treason features “intelligent action in a realistic thriller” in the tradition of Dan Brown's blockbuster Robert Langdon novels (TheMail on Sunday).