Season the guinea fowl pieces on all sides. Melt half the butter with the oil in a shallow flameproof casserole and fry the guinea fowl pieces for 5 minutes, until golden on all sides. Drain and reserve. Fry the sausages for 5 minutes, until browned all over. Transfer to a plate.
Add the remaining butter to the casserole and gently fry the carrots and leeks for 5 minutes until softened. Sprinkle in the flour and cook, stirring, for 1 minute. Remove from the heat and blend in the stock and wine. Stir in the juniper berries. Bring to the boil, stirring, then reduce the heat to a gentle simmer. Return the guinea fowl to the pan, cover and cook gently for 30 minutes.
Slice the potatoes thinly and arrange over the casserole. Season to taste. Cover with the lid or foil and cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 30 minutes. Dot the potatoes with the remaining butter and return to the oven, for 30 minutes until the potatoes are crisped and browned. (Pop the casserole under a preheated grill for a few minutes if desired.)