For the cucumber jelly, combine vinegar, sugar, 100ml water and a pinch of salt in a pan and bring to the boil, stirring until sugar has dissolved. Remove from the heat, and whisk in the gelatine until dissolved.
Roughly chop the unpeeled cucumber and whiz in a blender until pureed. Push the puree through a sieve into a bowl. Whisk in the vinegar mixture, pour into a container and refrigerate overnight if possible or at least 30 minutes.
For the pickled cucumber, finely slice the cucumber and toss in the rice vinegar, castor sugar and sea salt. Refrigerate for 30 minutes.
For the sauce, whisk the honey, mustard, vinegar, olive oil and dill until thick and runny.
To serve, finely slice the salmon and drain the pickled cucumber. Drape the salmon over four to six dinner plates or one large serving plate. Arrange teaspoonfuls of cucumber jelly and pickled cucumber casually on top and drizzle with the sauce. Scatter with dill.