Spit roasting is a real labour of love and the ultimate in barbecuing. This porchetta is made with a perhaps large cut to feed a big crowd - a whole side of pork, which includes the belly, loin and eye fillet - though you can easily halve the side to feed fewer people (the cooking time should remain the same). You'll need to order either cut ahead from your butcher. You'll also need an electric rôtisserie spit; some butchers have them for hire, or you can buy them at barbecue shops. The small electric ones can take up to 10kg of meat and that's what we used here. The key to spit-roasting is to distribute the weight of the meat evenly on the rôtisserie rod so it's balanced and turns easily. You may need to cover sections of the meat with foil as it cooks to protect it from the fat flare-ups.
Note For indirect grilling, arrange hot coals around the outside of the barbecue so the heat isn't directly under the meat.