Heat the oil in a large thick bottomed saucepan with a tight-fitting lid. Add half the garlic and as soon as it begins to colour add the pipis and half the wine. Cover the pan and cook just until the clams open. Strain using a colander set over a bowl to collect the liquid. Leave to cool. Take the pipis from their shells and return to the liquid.
Slice the white parts and the green parts of the leek finely, keeping them separate. Heat half the butter in a small pan, add the green leek, cover and cook for two minutes.
Bring the stock to a simmer and taste for seasoning. Add the prawns and cook for one minute. Remove with a slotted spoon and allow to cool. Shell and devein the prawns and add to the pipis. Add a few of the prawn shells to the stock and allow to simmer gently. When ready to use, strain out the shells and again taste for seasoning.
Heat a large thick-bottomed saucepan over a medium heat and melt the remaining butter. Add the onion and celery and cook gently, stirring to prevent sticking, When soft add the white of the leek, the remaining garlic, the fennel seeds, anchovies, two tablespoons of the parsley and cook together, stirring. Add the rice and stir to coat each grain, then pour in the remaining wine. Cook until almost reduced, then start to stir in the stock, ladle by ladle, stirring continuously, not adding any more stock until the previous ladle has been absorbed. Continue stirring and adding stock until the rice is almost cooked, about 15 minutes.
At the last minute, stir in the green leeks and their butter and the pipis and prawns along with their liquid. Stir to combine and allow the clam liquid to be absorbed. Quickly stir in the remaining parsley, the lemon juice and creme fraiche. The risotto should be quite wet and creamy in consistency. Serve immediately.