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Using a peeler, pare thin strips of zest from the orange and lemon. Squeeze the juice. Put the zests and juice into a large saucepan, then add the Port and remaining ingredients. Simmer very gently for 20 mins, then leave to cool completely. Can be made up to 1 week ahead and chilled. To serve, 'frost' the glasses if desired (see 'Tip', below left), put ice and slices of orange and lemon in each glass, then carefully pour in equal quantities of the Port and lemonade.