Unwrap your gammon. Strip off the netting. Pop into a large pot. Cover with water. Pop in the fridge and soak overnight. No time for overnight soaking? Just boil in water for 1 hr for a quick fix. This step removes excess saltiness.
Drain the soaked or par-boiled gammon. Dust all of the mixed spice over it. Pop in a clean pan. Cover with water. Pop on the lid. Boil for 1 hr. Pour out all but 250ml (or a 3cm deep puddle) of the spiced cooking liquid. Heat your oven to 200C/Fan 180C/Gas 6.
Peel and thinly slice your potatoes and apples (discarding seeds and stem). Lay them in a roasting tin and place the gammon on top, fat-side facing up. Pour in the spiced liquid from the pan. Pluck the sage leaves from their stems. Scatter them over the top. Dust the sugar over the top of the gammon. Roast for 45 mins or till the fat is a little golden and crackly on top.