Put a lid on the pan, put the pan onto medium heat and cook the rhubarb gently in the covered pan, for 10 minutes. As it cooks, the rhubarb seeps juice.
Meanwhile, soak 4 leaves of gelatine in cold water for 10 minutes. Lift the soaked gelatine from the water and drop them into the hot cooked rhubarb. Stir to dissolve the gelatine, then pulverise the contents of the rhubarb pan using a hand held blender. Cool a bit before dividing the rhubarb mixture evenly between 4 glasses. Leave till cold, and jelled.
For the syllabub:
300 mls double cream, whipped with
2 tablespoons syrup from a jar of preserved ginger and
1 tablespoon lemon juice
3 chunks of preserved ginger, each sliced into fine slivers.
Spoon the ginger syrup whipped cream, divided evenly, over each set rhubarb jelly. Put a small heap of sliced ginger on the centre of each.