Place the ingredients for the dipping sauce in a pan and bring to the boil. Then turn the heat off, add the bonito flakes (oigatsuo) and leave to cool - do not stir. Now strain and chill thoroughly in the fridge.
Meanwhile tie each portion of noodles into a bundle using rubber bands and cook in plenty of water according to the instructions on the pack. Strain the noodles and rinse in cold running water.
Once they are cool, remove the rubber bands using scissors and serve in a bowl of ice water. Pour the dipping sauce into individual serving pots and add yuzu zest as desired.
*This dipping sauce can also be used for udon noodles.