Make 300mls (½8pint) sauce, and mix with the prawns. Keep back 100mls (4oz) sauce. Mix the ricotta with the herbs, spinach and seasoning. Oil a rectangular dish, and build up layers of sauce, lasagne and ricotta, finishing with lasagne and sauce.
Bake for 20 minutes at 180°C/350°F/gas mark 4 in a pre-heated oven. Then spoon on the remaining sauce and prawn mixture, and return it to the oven for about 5-8 minutes. Serve hot and bubbling.