Soak the gelatine in cold water for 5 minutes, until softened.
Grate a little zest from 1 of each citrus fruit into a bowl and set aside.
Cut the zested fruits in half and squeeze the juice into a pan. Add 100ml of water, the Aperol, sugar and soaked gelatine. Heat gently until the gelatine and sugar have dissolved. Leave to cool.
Peel the remaining grapefruits with a sharp knife, remove the segments and place into the jelly mix.
Pour into 6 to 8 clean glasses or trifle bowls and place in the fridge for at least 3 hours, until set.
Pour the cream over the citrus zest and add the icing sugar. Beat until soft peaks form and then spoon on top of the set jellies.
Tips
This jelly is super versatile; if you don't fancy grapefruit, swap it out for your favourite citrus fruit, or even a zesty combo.