Put a saucer into the fridge to test for set later.
Thoroughly wash a large jam jar and place in the oven on an oven tray, set the oven to 100C.
Put all the ingredients in a medium sized pan.
Heat gently until the sugar has all dissolved.
Bring to the boil and keep at a rolling boil for 4 minutes (with jam sugar) or 8 minutes with granulated sugar.
Take off the heat and place a teaspoonful of the jam onto the cold saucer, put it back in the fridge for 5 minutes, then push the jam with your finger. If it wrinkles the jam is ready. If not put the jam back on the heat and boil for another 2-3 minutes before testing again.
Ladle into the jar and leave to cool for 10 minutes before putting on the lid.
Leave the jam to cool completely before use.
If you would like to use the jam more quickly, spoon some onto a plate and it will cool more quickly.