I've struggled for years with unreliable results, but this really seems to work:
Pre-heat oven to gas 6/200C/400F.
For four people take a 1-1.5kg (say 2 1/2 ibs) piece of boneless, skin on, belly pork. This will be about 10 inches square and 1 to 2 inches thick. Score the skin with a very sharp knife at about 1/2 inch spaces. Now cut the meat across and across again into 4 equal squares (so no carving!). Place meat, spaced a little, skin side up in a roasting tin and scatter chopped onion, carrot, celery, bay leaf, garlic etc around meat. Pour round boiling water to come about half way up the meat and stick it in the oven for 2 1/2 hours. Top up hot water if needed. Remove meat and place in another roasting dish, brush with olive oil and sprinkle over some salt. Back into the oven for 15 minutes. Serve.
The cooking liquid can of course be used to make gravy.
This was given to me by someone who said they had read it recently, so sorry if I've stolen it.