Preheat the oven to 220°C/200°fan/Gas 7. Mix the cheeses, mustard and tarragon and season with salt and pepper. Loosen the skin on the chicken breasts, spread the cheese mixture underneath then pull the skin back over. Place the chicken in a roasting tin with the potatoes. Drizzle with oil and roast for 25 minutes. Add the courgettes, tomatoes and olives and bake for a further 15 minutes. Put the chicken and vegetables onto four warm plates and keep warm. Add the sherry to the pan juices and place over a medium-high heat until reduced to about 4 tablespoons. Spoon over the chicken and serve.