A Death in Diamonds by S.J. Bennett

1957 - Young Queen Elizabeth II is finding her way in a challenging world as the United Kingdom must adjust to having neither an empire nor a set place in post-war Europe. As she travels the world to try and build bridges, the Queen is advised by the 'men in moustaches', as Philip calls them - her father's old courtiers, who may or may not have her best interests at heart. One of them is trying to sabotage her: that much she is sure of.

And then two bodies turn up, horribly murdered, in Chelsea, and the Queen finds herself unwillingly used as the alibi for somebody very close to her. Elizabeth knows she can't face these challenges alone. She needs support from someone clever, discreet and loyal, someone she can trust.

Then she meets ex-Bletchley Park code breaker, Joan McGraw, and their investigation into the murders begins in earnest . . .