Preheat the oven to Gas Mark 5, 190ºC, fan 170ºC. Cut the tops off the garlic bulbs horizontally, to expose the cloves inside. Place on a sheet of foil and drizzle with 1 tbsp olive oil. Wrap the foil round the bulbs in a loose parcel and bake for 40 minutes.
Boil the potatoes in salted water for about 18 minutes, until soft to the point of a knife. Drain and return to the pan for a minute over a very low heat to dry them out. Squeeze three-quarters of the roasted garlic cloves from their skins into the mash, with the milk or cream and remaining olive oil. Mash until smooth. Season. Serve with the remaining roasted garlic.