In a heavy saucepan, heat the sugar, water and lemon juice over medium-high heat. If you like, stir until the sugar dissolves. Otherwise, just swirl the pan occasionally.
Keep cooking it, swirling it occasionally, until it starts to turn golden. Don't leave it after this point - swirl the pan more often until it turns deep golden. Have the cream and butter ready and pull the pot off the heat and add them (or just cream) immediately as soon as it turns deep golden - it will spatter and steam. Stir until smooth - if there are any set chunks of caramel in the pot, they will melt back in. If you like, stir in a pinch of salt.
Cool completely and pour into a jar to keep in the fridge.