1. Place the frozen blackcurrants in a saucepan and add 100g of sugar. Allow to stand for one to two hours. This helps to draw some of the liquid out of the berries.
2. Add the port and spices and bring to the boil. Simmer for 10-15 minutes until the berries start to soften. Mix one teaspoon of pectin with approx. 20g of sugar. Stir into the boiling jam and test whether it has the required thickness. If it is still too thin, boil the jam for slightly longer.
3. Pour the jam into sterilised jam jars and seal immediately.