For curry paste, blend ingredients (except oil) in a small food processor until very finely chopped, then with the motor running, slowly drizzle in oil until a purée. Fry curry paste in a saucepan over medium heat until fragrant and excess water has evaporated (3 minutes).
Add coconut cream, soy sauce, sugar, fish sauce and a pinch of salt to saucepan and bring to the boil. Boil until thickened (5-10 minutes), then strain through a sieve and cool.
Cover chicken with 180ml curry sauce, cover and refrigerate to marinate (at least 2 hours or overnight).
Burn coals down on a charcoal barbecue to medium heat. Place chicken skin-side down on grill and cook, turning and basting with curry sauce occasionally, until cooked through (30-40 minutes; juices will run clear when a thigh is pierced with a skewer), then cut into 8 pieces.
Meanwhile, grill lemons cutside down over coals until lightly caramelised (2 minutes). Warm remaining curry sauce in a small saucepan over low heat (1-2 minutes). Pour over chicken and serve with lemon.
