Place the sugar, butter, lemon zest and strained juice in a heavy-based saucepan.
Place on a low heat and cook slowly until the sugar is dissolved and the butter has melted. Do not let the mixture boil. Remove the saucepan from the heat
Using a sieve, strain the eggs in to the mixture and stir well with a wooden spoon (one kept for sweet foods only).
Return the saucepan to the heat and cook the mixture over a medium heat until thick, stirring constantly.
Pour into hot, sterilised jars and allow to cool completely before placing on tight-fitting lids. Keep refrigerated. Lemon curd is best eaten within 10 days.