Nigel Tranter 
Mail Royal 

A casket of incriminating letters from Mary, Queen of Scots. The letters would be an instrument of persuasion – or blackmail as others might call it. Whoever held them had a hold over the James VI King of Scots, now also King James I of England.But for Andrew, seventh Lord Gray, laying hands on the letters was not going to be easy. They were secure in the half-ruined Fast Castle that clung to the sea-cliffs of Berwickshire. A castle that no one could enter if they were not welcome.And so young David Gray, his bastard nephew, was sent on a perilous mission . . . 'Through his imaginative dialogue, he provides a voice for Scotland’s heroes’ Scotland on Sunday