Heat your oven to 240°C/Fan 220°C/Gas 9. Unwrap the pork and pat it dry with kitchen paper. Set it in a roasting tin, skin side up, and set aside for up to 1 hr to get it up to room temperature.
Finely chop the rosemary leaves and pop them in a bowl. Peel and crush or grate the garlic cloves and add them to the bowl with a good pinch of salt and pepper.
Tip 2 tsp fennel seeds into a pestle and mortar and grind them as finely as you can. No pestle and mortar? Use a small bowl and a jam jar or the end of the rolling pin to roughly crush them.
Stir the fennel seeds into the rosemary and garlic. Add 2 tbsp olive oil and stir to make a paste. Massage the paste all over the pork, making sure to rub it into the slits in the skin and in any gaps at the ends of the rolled pork.
Roast the pork leg for 25 mins, then reduce the oven to 180°C/160°C/Gas 4. Roast for another 1 hr.
Take the pork out of the oven – it should be meltingly soft with crispy skin. Pop the roast pork on a board or warm plate and rest for at least 30 mins before carving. If the crackling isn’t quite crunchy enough, slice it off in one big piece and place on a foil lined tray. Grill for 2-5 mins till the crackling is crunchy. Meanwhile, loosely wrap the pork in foil while it rests.
Carve the pork into thick, juicy slices and serve with shards of the crackling.