Preheat the oven to 190c/Gas 5 then put the leeks and apples into a roasting dish that will hold all of the sausages in a single layer. Place the sausages on top of the leeks and apples and drizzle over The Village Press olive oil, season and toss together, leaving the sausages on top. Dot over the butter and pour over the cider, then bake for one hour. Spread the mustard over the sausages and return to the oven for 15 minutes. Serve with mashed potatoes. Serves 4.