Preheat the oven to 180°C, 350°F, Gas 4. Mix together the celery, pecans, prunes, pepper and sage in a small bowl. Make a slit in each chicken breast and stuff each one with the prune mixture. Stretch 2 slices of pancetta with the back of a knife, place a chicken breast on top and wrap the pancetta around it. Repeat with the remaining pancetta and chicken breasts. Place the stuffed breasts on a non-stick baking tray and drizzle with olive oil. Bake in the preheated oven for 20-25 minutes until cooked through. Serve with a green salad or vegetables of your choice.