Heat sugar, vinegar, and 100ml water in a saucepan until the sugar dissolves.
Add chilli, garlic, and ginger, boil for three minutes, then cool in bowls to thicken.
Blend crabmeat, prawns, curry paste, cornflour, salt, and pepper in a food processor until just combined.
Transfer to a bowl and stir in spring onions and chopped coriander.
Shape the mixture with wet hands into 12 cakes approximately 1.5cm thick and 6cm wide.
Dust the cakes with extra cornflour on a tray.
Fry cakes in hot sunflower oil for two minutes on each side until lightly browned.
Serve crab cakes with jam, lime wedges, and coriander sprigs.