To make the dressing; heat the stock in a saucepan, add the orange zest and juice, tarragon, oil and vinegar. Simmer for 2-3 minutes. Chill for at least 2 hours.
Just before serving, place the salad leaves, turkey and grapefruit pieces in a serving dish. Pour the dressing over and toss well. Garnish with sprigs of tarragon.