Thinly slice the cucumber into ribbons and marinade in the white wine vinegar, sugar, salt, coriander seeds and a dash of water for at least half an hour.
In the meantime, mix the crab meat, most of the lime juice, chopped coriander, chilli and crème fraîche together in a bowl.
Dice the avocado and in a separate bowl mix with the rest of the lime juice, crushing it all together.
To serve, lay a quarter of the cucumber ribbons onto each plate. In a (roughly) 7cm cooking ring place a quarter of the avocado and spread evenly. Layer a quarter of the crab mix on top. Remove the ring and serve with brown (or gluten free) bread and butter if you wish.