Shell prawns. Melt butter add curry powder, chopped onion & capsicum. Peel and chop tomato, add to pan, cook for 2 minutes. Stir in flour, salt & pepper. Cook for 1 minute. Gradually add water & milk, add crumbled stock cubes, continue stirring until sauce thickens & boils. Reduce heat, add sugar & lemon juice. Simmer for 20- 30 minutes, stirring occasionally. Add prawns, allow to heat through. Serve with hot rice.