Place the blackcurrants, sugar and water into a pan over a medium heat and cook gently for ten minutes, stirring regularly, or until the berries are beginning to break down.
Strain the pan contents through a sieve into a clean pan, squeezing the berries through the sieve using the back of a wooden spoon to form a purée. This should yield approximately 300ml/10½fl oz of juice.
Add the crème de cassis, stir well and set aside.
Meanwhile, in a bowl, cover the gelatine sheets with cold water. Set aside for five minutes to soften.
Meanwhile, heat the cream and milk in a pan until it is almost simmering, then remove from the heat. Squeeze any excess water from the soaked gelatine sheets, then add them to the hot cream mixture, stirring until they dissolve.
Stir the cream mixture into the blackcurrant mixture until smooth and well combined (taste, then stir in a little extra sugar if necessary).
Rinse a 1 litre/1¾ pint jelly mould or pudding basin with cold water. Pour the blackcurrant cream mixture into the prepared mould and cover with cling film. Chill in the fridge for 7-8 hours, or until set.
Remove the cling film and dip the mould into a large bowl of just-boiled water until the water reaches nearly all the way up the sides. Suspend the bowl in the hot water for five seconds (no longer), then, using your fingertips, press around the edge of the blackcurrant cream to break the seal. Invert the pudding basin or jelly mould onto a serving plate and tap the top of the mould. (NB: The blackcurrant cream should be released onto the plate - if not, repeat the process.)
To serve, cut the blackcurrant cream into slices. Garnish with the remainins blackcurrants and drizzle over the cream.