Cook the potatoes and swede in a large pan of boiling salted water for 10 minutes. Add the carrots and cook for a further 5 minutes until both are tender. Drain, then return to the pan and lightly break up with a fork.

Heat the oil in a large non-stick frying pan, add the leeks and fry for 2 minutes. Add the potatoes, swede and carrots continue to cook for 4-5minutes, stirring occasionally until the mixture has begun to brown slightly. Stir in the watercress and cook for 2 minutes more until wilted. Season to taste.

Meanwhile cook the sausages under a hot grill for about 10 minutes, turning until browned all over and cooked through. Serve the bubble and squeak hot with the sausages and onion gravy if liked.