Pre-heat oven to 180ºC / 350ºF / Gas mark 4. Place the sausages in a large ovenproof dish and cook in the oven for 15-20 minutes, shaking half way though, until evenly browned and cooked through.
Meanwhile, place the potatoes in a large saucepan, cover with boiling water and boil until tender. Drain, return to the pan and heat gently to remove any excess water. Add the milk and mash until fluffy. Stir in the Schwartz for Chef Wholegrain Mustard and keep warm.
Make the gravy according to instructions or from scratch. Add the Schwartz for Chef Cumberland Sauce with Port, stir through and keep warm. Place a large pile of mashed potato on a plate, with three sausages and drizzle over the gravy.
