Scrape the white pith from a 10 cm (4 in) length of orange rind, and cut into thin matchsticks. Combine rind and 1 cup of water in a small pot and bring to the boil. Simmer gently for 15 minutes. Add redcurrant jelly and heat through gently, stirring. Add mustard powder, port, orange juice and lemon juice. Cook gently for 5 minutes, stirring well. Strain into a small bowl and allow to cool.