Put the kombu and water into a small saucepan and bring to the boil, stirring constantly to release the flavour of the kelp.
Remove from the heat and drain the water into a second saucepan, discarding the kombu pieces.
Add the katsuobushi flakes and set the saucepan over moderate heat.
Bring to the boil and boil for 1 minute.
Remove the pan from the heat and set aside until the flakes settle.
Strain the liquid, discarding the flakes.
The stock is now ready to use.