Combine mayonnaise, McCormick dill, chilli flakes, salt and lemon juice. Mix well.
Whisk the egg and the soda water. Add in the flours and mix.
Heat the oil in a medium sized frypan until hot (220°C). Coat each prawn in some of the extra flour and then dip in the batter.
Shake off any excess batter and deep fry prawns in batches in the oil until cooked through and crispy on the outside (about 2 minutes each). Pat the prawns dry with paper towel and serve hot with the dip.