Sterilise 4 370g jars and lids (or 5 slightly smaller ones) by washing in hot soapy water, rinsing well then placing them in a low oven until dry.
Place all of the ingredients apart from the port into a large pan, heat gently, stirring, until the butter has melted then simmer very gently, stirring occasionally, for about 10 minutes.
Remove from the heat and leave to cool until just warm then stir in the port. Divide the mincemeat between the sterilised jars, seal, and store in a cool place for up to six months.