Butterfly each shrimp by cutting along the back, through the shell, to remove the black vein. Rinse, then spread each shrimp wide open, leaving the shell on. Sprinkle with salt and the freshly ground black pepper. Set aside for 1/2 hour. In a large, heavy pan, with a tight-fitting lid - preferably a large wok - heat the olive oil over medium-high heat. Season the chicken thighs well with salt and pepper, then add them to the hot oil, skin-side down. Cook for 2 minutes, or until the skin just begins to brown. Add the chorizo slices, and toss them with the chicken. Cook for 1 minute. Add the shallots, garlic and string beans. Mix well. Add the rice and half of the parsley, stirring to coat the rice thoroughly with oil. Add the chicken stock, saffron and paprika. Stir well. Bring the mixture to a boil, reduce the heat, cover, and simmer for 15 minutes. Lift the cover, embed the shrimp in the top part of the rice, then scatter the snails around the shrimp. If you are using the red pepper, cut it into 8 strips and arrange it in the rice in a decorative pattern. Cover, and cook for 10 minutes more, or until almost all the liquid has been absorbed and the rice is just cooked. Serve at the table out of the cooking vessel, sprinkling the portions with the remaining parsley.