Mail royal - Nigel G. Tranter

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