Lightly oil a roasting tin and put in a 240c/fan 220c/gas 9 oven to heat. Season the skin of the pork loin with sea salt, then put in the hot tin and roast for 20 mins. Reduce the heat to 190c/fan 170c/gas 5 and roast for a further 30 mins per 500g. Increase the heat to 240c/fan 220c/gas 9 and cook for a final 10 mins to get a really crisp and golden crackling.
Remove the pork from the tin to rest, and pour off the excess fat. Cut the apples into wedges, toss them in the remaining fat in the tin and roast for 10 mins, then serve with the pork.