To make the posset: place the cream, sugar and lime zest in a pan and gently heat until the sugar is melted. Add the lime juice and strain through a fine sieve into 4 glasses. Chill overnight.
Put the sugar, water and vanilla in a pan and bring to boil. Set aside and add half of the rhubarb, thinly sliced, and let marinate for 1 hour.
Pick the marinated rhubarb from the syrup and set aside. Chop the rest of the rhubarb and place in the syrup. Bring to boil and add the soaked gelatine leaves, blend and strain. Let it cool down before serving.
Prepare the clotted cream mousse by whipping all the ingredients together until soft peaks and refrigerate for 1 hour.