Put the blackcurrants in a pan with a splash of water. Cook on a low heat for 5-10 minutes until the fruit has released its juices.
Zest the lemon into a small bowl and then squeeze the juice into the same bowl.
Place a heatproof bowl (I used a pudding bowover a pan of just simmering water. Put the lemon juice and zest, butter and caster sugar in the bowl and heat gently until the butter has melted and the sugar has dissolved.
Set the bowl aside while you prepare the eggs. Crack the eggs into a bowl, whisk well with a fork and then sieve the eggs into the pudding bowl over the water.
Add the fruit from the other pan, stir well, and keep stirring for 30-40 minutes until thickened and it coats the back of a spoon.
Strain the fruit out using a sieve and pour into a sterilised jar.