Melt the jelly with half the amount of water recommended on the packet, allow to cool at room temperature for 30 min.
Using an electric whisk, whisk the evaporated milk until it has tripled in size. While continuing to whisk gradually add the jelly. Whisk for a further 30 seconds then pour into a bowl. Put in the fridge to set.
Warning: when whisking use a bigger bowl than you think you need as you'll be surprised how much this fluffs up.