Place the egg yolks in a heatproof bowl that will snugly fit over a saucepan of simmering water (make sure the bottom of the bowl doesn’t touch the water). Add most of the lemon juice then, very slowly, start to pour in the butter, whisking continuously. As the butter thickens the sauce, you can add it a little quicker. When the sauce has thickened and is the consistency of mayonnaise, remove from the pan. Leave to cool, season well and add more lemon juice to taste. Whip the cream until soft peaks form, then carefully fold into the sauce.
Meanwhile, cook the pasta in a large saucepan of salted boiling water according to the pack instructions until al dente.
Heat the oil in a saucepan, add the leeks with a splash of water and cook gently for about 7 minutes until beginning to soften. Add the prawns and cook for 2 minutes or until heated through.
Drain the pasta and return to the pan, then mix through the sauce, leeks and prawns and season. Spoon into individual gratin dishes and cook under a preheated hot grill for 3–5 minutes or until lightly browned all over.