It keeps in the fridge for a month or so. When you use it, add a squeeze of lime and a handful of coriander leaves for a fresh boost.
Using a large mortar and pestle, add the salt and garlic and pound to a paste. Add the chilli and pound to a rough paste, then tip into a small saucepan. Place the dried shrimp in the mortar and pound until they break down a little and become fluffy. Add them to the saucepan along with the sugar, vinegar and celery seeds.
Place the saucepan over a medium heat and bring to a simmer for 5 to 8 minutes or until slightly syrupy. Add the fish sauce and simmer for 1 minute then tip into a clean jar and allow to cool. You will end up with approximately 260 to 280 ml of sauce.