Place the blackcurrant in a blender and add the water and lemon juice.
Using a handheld blender, mix the mixture to a fine puree and pass through a fine sieve to catch any woody pieces.
Transfer the mixture into a bowl and place over a saucepan of simmering water, over low / medium heat. Wisk in the caster sugar and eggs.
Whisk continuously, by hand , for 5 minutes or so until it thickens and then slowly whisk in butter.
Transfer into serving cups or store in a container to use at a later date. Once cooled, cover with cling film and store in the refrigerator. Use within 5-6 days.