Carriage of Justice — Gerald Hammond

In the Scottish Highlands, foxes are a gamekeeper's worst enemy. Except, of course, for poachers. At first Deborah Fellowes and her Uncle Ronnie think they've caught the villain, but it soon emerges that they're on the trail of someone far more sinister.
So when a startled trespasser leaves behind a surprising set of fingerprints and a shotgun stolen years earlier, they realize they have uncovered something dangerous indeed: their "poacher" may turn out to be the perpetrator of a notorious, long-unsolved murder.