Slice the bulb of garlic in half horizontally. Place on a square of tin foil and drizzle with a little oil, then seal the foil tightly and roast in the oven for 45 to 55 minutes, until the garlic has softened.
Meanwhile, peel the potatoes and place in a large saucepan of water. Bring to the boil, then reduce the heat to a simmer, cover with a tight-fitting lid and continue to cook for 20 minutes, until the potatoes are softened.
Drain the potatoes in a large colander and return to the saucepan with the butter and milk. Squeeze the roasted garlic out of its skins and into the potatoes. Mash the potatoes until completely smooth, then season with salt and pepper, stir through the parsley and serve immediately.