Boil the kettle and let the water cool down to just warm. Dissolve the citric acid in 60ml of the water. Pour the milk into a large saucepan and heat gently to about 13C. Add the citric acid, and heat to 30C, stirring gently – it will start to curdle.
Dilute the rennet in a tablespoon of the boiled, cooled water and add it to the milk. Warm gently to 38-39C, stirring from time to time – it will begin to separate. Remove from the heat, cover and leave for about 15 minutes, to let the curd set and separate from the whey even more.
Carefully scoop the curds out of the pan with a slotted spoon and place them in a sieve – leaving the hot whey in the pan. Press the curds very gently to remove some of the whey, but not all of it or your mozzarella may be tough. Add the salt to the whey in the pan and heat to 80C.
Mound the curds onto a chopping board and cut into 4 pieces. Have a bowl of iced water ready. One at a time, dip the curds into the hot whey for about a minute, until they begin to soften and stretch. Remove a piece of curd from the whey with a slotted spoon, dip your hands in the iced water and gently stretch out the cheese, folding it back on itself and working it just until it's stretchy, shiny and smooth.
Mould and pinch the cheese into a ball then drop it into a bowl of chilled water. Repeat with the remaining curds, refrigerate and use within two days.