(1) The most authentic are Candle nuts but I've never seen them for sale in the UK. Substitutes are Macadamia nuts, Brazil nuts, Cashews, or blanched Almonds. Candle nuts are toxic unless cooked well.
(2) for 1/3 cup of tamarind juice, soak 1 teaspoon pulp with 1/3 cup of hot water for 5 min, mash, and strain