Complex sentence: Leuchars was not to secure the romantic image of a Battle of Britain station, but rather settled to the routine of hour upon hour of maritime patrol which played a crucial part in Britain's ultimate victory.
Split into simpler sentences: Leuchars was not to secure the romantic image of a Battle of Britain station but rather settled to the routine of hour upon hour of maritime patrol. The contribution such unglamorous work made to the war effort should not be underestimated, and such patrolling played a crucial part in Britain's ultimate victory.