In a skillet, melt butter and add curry powder, celery, onion and capsicum.
Saute gently for 2-3 minutes. Add tomato and cook for a further 2 minutes.
Stir in flour, salt and pepper and cook for 1 minute.
While stirring, gradually add water and milk, then add crumbled stock cube.
Continue stirring until sauce boils and thickens.
Reduce heat, add sugar and lemon juice and simmer for 20 minutes, stirring occasionally.
Add prawns and heat through, being careful not to overcook.
