Melt butter in a large skillet set over medium heat.
To the melted butter add the diced onions, celery, and peppers; saute for 2 to 3 minutes, or until beginning to soften.
Stir in the garlic, curry powder, and paprika; cook for 15 seconds.
Add the tomato and continue to cook for 2 minutes.
Stir in the flour, salt, and pepper; stir and cook the flour for a minute.
Whisk in the chicken broth and milk until smooth; bring to a boil.
Reduce heat to a simmer; whisk in lemon juice and sugar and simmer for 20 minutes, stirring occasionally.
Add prawns to the sauce and cook for 2 minutes, or until pink and cooked through.
Taste for salt and seasonings; adjust accordingly.
Serve over rice and garnish with cilantro and basil.