Peel and devein the prawns, reserving the shells and heads. Chop the prawns into bite-size pieces.
To make the prawns stock combine the prawns shells and heads, onion and celery in a large pot. Squeeze the lemon juice into the pot then add the lemon halves as well. Add the dried basil, oregano, thyme, peppercorns, 1/2 cup parsley and 3 cups of water.
Cover the pot and bring to a boil. Reduce heat and simmer uncovered for 1 hour, skimming away any foam or impurities that have risen to the surface. Strain the stock and set aside.
Heat the butter in a large frypan over medium heat. Cook the prawn pieces until pink and almost cooked through, 1 to 2 minutes. Remove and set aside. Add the eschallot to the pan; cook and stir until softened and translucent, about 5 minutes.
Stir in the garlic, lemon zest, salt and ground pepper. Cook for thirty seconds then add the white wine. Bring to a boil and cook until the white wine is reduced by half, about 5 minutes. Add the stock to the pan, bring it to a boil and simmer until the sauce is reduce by half, about 15 minutes.
Meanwhile, fill a large pot with heavily salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in ravioli and return to a boil. Cook uncovered, stirring occasionally, until the ravioli float to the top and the filling is hot, 3 to 4 minutes. Drain well, reserving a little pasta-cooking water to thin the sauce if necessary.
Stir the diced tomatoes and cream into the sauce and heat through. Return the prawns to the pan, taste the sauce and season to taste with salt and pepper. Fold the ravioli into the sauce.
Place four ravioli onto each of four warmed pasta bowls or plates and top with sauce. Garnish the pasta with chopped parsley and lemon zest.