Other Paths to Glory by Anthony Price

A First World War battlefield hides a deadly secret - one that some are willing to kill for 

Paul Mitchell is a young military historian whose life is changed forever when two men, Dr Audley and Colonel Butler of the MOD, visit him with a fragment of a German trench map - and a lot of questions. Then somebody tries to kill him. Paul, his life now in danger, agrees to go underground on a mission to solve a dangerous mystery: what really happened during the battle of the Somme in 1916? And why does somebody want to keep it secret?