Tip the berries and clementines into a preserving pan, and pour in 1 litre of water. Gently heat, stirring frequently, for 30 mins until the berries are soft. As the mixture softens, lightly crush to release all the juice from the berries. You can do this with a potato masher.
Cool the mixture, then tip it into a jelly bag suspended over a large bowl, or into a large plastic sieve lined with muslin. You need a plastic sieve, as the acidity in the fruit will affect anything metal. Leave overnight so that the juices run through.
The next day, measure the liquid. You should end up with 1.4 litres. If you have less, make up the mixture with water. If more, boil to evaporate the excess. Meanwhile, sterilise your jars and put a small plate in the freezer to chill.
Return the liquid to the pan and add the sugar.
Heat until dissolved, then bring to the boil, skimming off any scum that rises to the surface, and allow to cook for about 20 mins. Use a sugar thermometer to take the temperature up to 105C or, after 15 mins, start checking its set by dropping the mixture onto a chilled plate. When it is ready, the mixture will appear set when you push your finger through. The drips that come off the spoon will also look syrupy. Spoon off any remaining scum and stir in the Port. Pot into the jars, then leave to set. Will keep for at least a year.
