Using a sharp knife, remove the membrane or flap from the bony side of the ribs. Mix the oil, garlic, spices and salt, then rub this all over the ribs. Allow to marinate for at least 1 hour but overnight is best.
Meanwhile, make the tamarind glaze. Break the tamarind into small pieces, place in a heatproof bowl and pour over the boiling water. Leave to soak for about 30 minutes, mashing it a bit every now with your fingers and removing any seeds or large fibres as you go. Strain through a sieve into a pan to remove any remaining fibres and seeds. Add the sugar and remaining ingredients to the pulp, then gently cook until the sugar is dissolved, adding more sugar, salt or chilli to taste.
Get your barbecue up to a target temperature of 110–130°C. Place the lamb on the grill, close the lid and cook for 3 hours until soft, brushing with 2 tbsp of the tamarind glaze during the last hour.
Remove the lamb and rest for 10 minutes while you increase the temperature of your barbecue to 220–250°C. Cut the lamb into ribs, brush with a little more glaze and then crisp on the grill with the lid closed for 2–5 minutes on each side, until sticky and beginning to char.