Preheat the oven to gas 3, 160°C, fan 140°C. Place the gammon in a large roasting tin, skin-side up. Nestle the onion, celery, peppercorns and herbs all around it, then pour 500ml (17fl oz) water into the tin. Cover the ham with a sheet of nonstick baking paper, then cover the whole tin with a tent of foil. Bake the ham for 5 hours, or until a meat thermometer reads 71°C when inserted into the middle of the joint. Meanwhile, use a pestle and mortar to crush the chillies to a rough paste. Place the chilli paste, maple syrup and sugar in a medium pan. Remove the gammon from the oven, uncover and let it cool a little. Drain off most of the liquid (keep this as stock for soup - it can be frozen if you like), reserving a little in the tin. Snip off any string then cut away the skin, leaving as much of the fat on as possible. Score lines (about 4cm/1½in apart) into the fat with a sharp knife to make large diamond shapes, being careful not to cut right through into the meat itself. Turn the oven up to gas 6, 200°C, 180°C fan. Gently heat the glaze ingredients until the sugar has melted, then boil for 1 minute. Use a pastry brush to paint 2/3 of the glaze over the fat, working the chopped chilli well into the cuts. Roast the ham, uncovered, for 45-50 minutes, brushing with the remaining glaze during cooking, until it's golden and caramelised (if the glaze thickens up, just warm it briefly). Serve hot or cold. Tip Depending on how your gammon has been cured, it may need soaking overnight in cold water to remove any excess saltiness; ask at the meat counter for instructions. The recommended gammon joint is avaliable at the meat counter in your local Tesco store.