Place the vinegar, sugar and finely sliced red onion in a small bowl, mix well and leave for at least 30 minutes. In another bowl mix the crab meat, crème fraîche, paprika, horseradish and shredded spring onions. Add a squeeze of lemon juice, then season and spoon into Kilner jars or ramekins. In a pan, melt the butter, add the cayenne pepper then pour a thin layer into the jars or ramekins to cover the crab. Chill for 30 minutes until set. When ready to serve, drain the red onion, toss with the sliced radishes, watercress and peppercorns and pile over the crab meat and butter. Serve with sliced toasted ciabatta and lemon wedges for squeezing.