Preheat the oven to 425 degrees F.
Loosely wrap the sweet potatoes individually in aluminium foil and cook in the oven for 1 1/2 hours or until cooked through. Leave to cool in their foil parcels.
Meanwhile, peel and chop the white potatoes and cook them in salted water until tender; then drain and mash.
Open the sweet potato parcels carefully over the mashed white potatoes to catch all the juices that have leaked from them. Still holding each sweet potato over the other potatoes, peel the skin away with your fingers and scoop out the flesh into the mash.
Add the oil and mace and beat the potatoes together very well with a wooden spoon adding salt and pepper, to taste. Serve immediately.