Place all ingredients, except the potatoes, in a large saucepan & bring to a boil. Cover, reduce heat & simmer until the vegetables & barley are tender, about 45 minutes. Preheat the oven to 180ºC . Transfer the cooked mixture to a lightly oiled casserole dish & top with alternate diagonal rows of both kinds of potato. (For a more decorative look, try piping the potatoes). Bake for 35 to 40 minutes or until the edges bubble & the potatoes are golden brown. Serve immediately.