1) Make the crayfish sauce by placing the heads and claws in to a saucepan with enough water to cover them, add the peppercorns and bay leaves. Bring to a simmer and cook for 25 minutes. 2) Add the tails and cook for 2 minutes more. 3) Strain the cooking liquid into a clean pan, add the cream then bring to the boil and reduce by a third. 4) Add the watercress and mustard powder and cook for a minute more. 5) Blend until smooth with a hand blender. 6) Remove the crayfish tails from the saucepan and remove the flesh. Discard the heads and claws. 7) Heat the oil in a frying pan over a high heat and fry the pike fillets, skin side down for 3 minutes or until the skin is golden. Turn and fry for 3 minutes on the other side. 8) Meanwhile heat the butter in a large saucepan over a medium heat. Add the sorrel leaves and cook for about two minutes until wilted. 9) To serve, divide the sorrel between 4 plates, top with a fillet of pike and pour the sauce over. Add 3 crayfish tails to each serving.