Combine garlic, ginger, sauce, sherry, pepper and oil in large bowl; add chicken, coat in marinade and cover and refrigerate overnight.
Next day - preheat oven to 220C/200C fan forced (430F/390F fan forced).
Half fill a baking dish with water and place an oiled rack over the baking dish and place the chicken on the rack.
Roast the chicken uncovered for about 1 hour and 20 minutes or until cooked through (I would keep an eye on the water level and possibly replentish if looking at drying out).
While the chicken is cooking make the shantung sauce - Combine sugar and the water in small saucepan, stir over low heat until sugar dissolves and bring to a boil and let boil for about 5 minutes (uncovered) WITHOUT STIRRING until sauce thicken slightly.
Remove from the heat and stir in vinegar and chilli.
Remove chicken from the oven, when cool enough to handle, remove bones and chop meat coarsely and serve drizzled with the shantung sauce (do a gentle reheat of sauce if required).