Cook the blackcurrants with the water and (golden granulated) sugar in a large saucepan until tender.
Dissolve the cornflour in a little water and add to the pan. Stir until thickened and leave to go cold.
Melt the butter, add the demerara sugar and stir well. Add the rolled oats and mix well to coat. Cool.
Spread half of the oat mixture into a glass serving dish. Spread the blackcurrants over the oat mixture then spread the remaining oat mixture on top of the blackcurrant mixture.
Spread whipped cream over the oat mixture and chill. Scatter over grated chocolate and place a spring of mint in the centre.