Trim the rhubarb discarding the tops and the bottoms. Wash and cut into very small chunks. Place in a non corrosive pan with the sugar and water.
Bring up to the boil, then cover and simmer for 10 minutes or until the rhubarb is very soft.
Meanwhile put 3 tbsps of very hot water into a small bowl and sprinkle with the gelatine. Leave for 5 minutes then stir. If it does not completely melt then place the bowl in a shallow pan of simmering water until dissolved.
Strain the rhubarb and set aside the pulp to make a fool. Measure the rhubarb syrup- you will need a pint so add extra water if necessary. Stir in the Cointreau.
Mix the melted gelatine into the warm syrup. Transfer to a large covered bowl, cool then chill until the jelly is nearly set.
Using a large whisk, vigorously beat the jelly until it bubbles up into a frothy mass. Pour into four wineglasses, lightly cover and leave to set in the fridge.