Cut nori in half. Cut one half into 5cm squares. Cut the other half into 2cm wide strips. Divide harusame noodles into small bundles of 6-8 noodles each. Brush seaweed strips with water and wrap tightly around the middle of each bunch of noodles.
Heat oil in wok or deep fryer to moderately hot, 170 deg C (340 deg F).
Using chopsticks and being careful not to overmix, prepare batter by mixing flours with lightly beaten egg and enough cold water until just combined but small lumps of flour can still be seen.
To test oil temperature, drop a small amount of batter into the oil. If it quickly floats, it is ready. Dust prawns and vegetables, but not nori, with potato starch, shaking of excess. Dip in batter and cook, in batches, until golden. Nori is only dipped in batter on one side.
Drain on paper towel. Allow oil to come back to correct temperature between batches.
Increase oil temperature to 200 deg C (400 deg F) and cook noodles in batches for 1-2 minutes or until they double in size and float. Serve immediately as a garnish with tempura and lemon wedges, with dipping sauce and daikon and ginger separately.
Dipping sauce: Combine sauce concentrate with warm water. Pour into 4 dipping bowls. Place grated daikon in another small bowl and top with grated ginger. Daikon and ginger are added to the dipping sauce, according to personal taste.
