The trick to a killer roast is your roasting pan. Make sure the bottom of your roasting pan is flat and about 1/4cm thick. That will give your potatoes a super crispy bottom.
New Zealand yams are only available in New Zealand and South America. They are sometimes known as oca and are a small vegetable with the flavour of a lemony parsnip. If you can not get yams, use parsnip instead.