Make the sauce up to two days in advance, reheat and add the fish to finish. You could use monkfish to make a more luxurious curry or use chicken thigh for a meaty option as the sauce goes very well with chicken. You could also just use prawns, it is very adaptable.
If you don't have or can't get tamarind then use a mixture of lime juice and brown sugar until you get a paste that is both sweet, sharp and tangy erring on the side of sour and add that to the base. If you don't have lime use lemon. Another substitute could be pomegranate molasses if you have a bottle lurking.
Serve with basmati rice simmered in half coconut milk, half water with a stick of cinnamon and a bay leaf for extra flavour.