Line your roasting dish with tin foil, then chuck half of your rosemary (peel the leaves of the stalk) in the bottom, pop the pork on top (fat side up), then put the other half of the rosemary on top of the pork. Cover your roasting dish with tin foil.
Put the pork in the oven and turn the temperature down to 220C, and set a timer for 40 minutes.