With a sharp pair of kitchen scissors cut the legs and snout off the prawns.
Rinse and set aside.
In a large sauce pan, heat the oil over medium-high heat.
Add the flour.
Cook, stirring constantly, until the flour and oil are well combined.
Remove from heat and add the leeks, mixing well to coat.
Return to the stove, reduce heat to low and cook, stirring often, until the leeks become soft (about 5 minutes).
Stir in the parsley and garlic.
Cook, stirring constantly, 1 minute more.
Stir in the tomato puree, the tomato paste, clam juice, water, vinegar, salt and pepper.
Simmer, uncovered, for 30 minutes.
Stir in the prawns and simmer, uncovered, 12–20 minutes more, or until the prawns are fully cooked.
Serve over cooked rice, pasta or polenta.