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.
Peel and crush or grate the garlic and ginger into a small bowl. Stir in 2 tsp ground cumin and a good pinch of salt and pepper.
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 meat.
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.