Stir the tomato puree into 120ml hot water until completely smooth. Heat the base curry sauce until steaming and divide it into 200ml, 200ml and 100ml portions. Place all the measured ingredients beside the hob.
Heat the oil in a wide pan with a capacity of at least 3 litres over a medium high heat. Add the cardamom pods, star anise and bay leaf. Stir for about 30 seconds, until aromatic but not darkened.
Add the Garlic and Ginger Paste, bird's eye chillies and optional Scotch bonnet. Stir continuously for 30 to 45 seconds, until the raw aroma begins to soften. Reduce the heat if the paste starts to colour.
Reduce the heat to medium. Add the Mixed Powder, turmeric, Kashmiri chilli powder and optional hot chilli powder. Stir continuously for 20 to 30 seconds, just until fragrant.
Pour in the diluted tomato puree and stir thoroughly. Add the first 200ml portion of hot base curry sauce, bring to a lively simmer and cook for 2 minutes.
Add the pre-cooked lamb and the second 200ml portion of base curry sauce. Simmer for 4 to 5 minutes, turning the lamb through the sauce so it reheats evenly.
Fold in the pre-cooked potatoes, vinegar and crushed dried fenugreek leaves. Add up to the final 100ml portion of base curry sauce as required. Simmer for 4 to 6 minutes, stirring carefully, until the lamb and potatoes are steaming hot throughout and the sauce coats them without becoming dry.
Stir in the salt and 15g of the coriander. Taste and adjust the vinegar, salt or chilli if required. Remove the star anise, bay leaf and any visible cardamom pods, then garnish with the remaining 5g coriander and serve.