Heat the oil in a heavy based saucepan and cook the onions for 10 minutes or until soft. Add the chilli (if using), garlic, ginger, tikka masala paste and red pepper and cook for a further 5 minutes. Cut the potatoes into 2-3cm chunks and add the raw potato to the pan. Stir well before adding the tomato purée, tinned tomatoes and water. Bring to the boil, then reduce heat and simmer for 10 minutes, or until the potato is cooked through. Add the chicken, seasoned with salt and pepper, and cook for another 15 minutes (top with a little water if the meat doesn’t fit beneath the surface of the liquid). Add the cream and season well. Serve with chopped coriander, flaked almonds and lime wedges.