Put the prawns in a bowl with half the chilli flakes and half a teaspoon each of salt and pepper, and leave to marinate for an hour.
Heat the oil in a wok on medium heat, add the cinnamon, ginger and garlic, and stir-fry until the garlic turns golden brown. Add the remaining red chilli flakes, green chilli and curry leaves, and take off the heat.
Stir in the tomatoes, paste and ketchup, put back on the heat and stir-fry for three to four minutes. Add the vinegar and spring onions, and stir-fry on a medium heat for six to eight minutes, until the sauce is thick and glossy.
Add the onions, green peppers and prawns, stir-fry on high heat for five to seven minutes, until the prawns are pink and cooked through, then add sugar and salt to taste.
Tip out on to a platter, garnish with chopped fresh coriander and sliced spring onions, and serve hot.