Place all ingredients in a large, heavy pan. On a medium heat stir until the sugar has melted.
Reduce the heat to as low as possible and leave to simmer for another 2 - 2½ hours.
Note - During the first hour I did not have to do anything - no checking and no stirring. The next ½ an hour I just kept a close eye on the sauce and stirred every now and again. For the last ½ an hour I stirred almost constantly to prevent the sauce from sticking to the pan. For the consistency that you see in the photos I stopped when the sauce was still quite runny in the pan (100° C on a sugar thermometer). It firms up once it has cooled down.
Will keep in the fridge for over a month. It may need to be warmed up slightly to soften before use.