Put the vinegar, mirin and caster sugar into a small saucepan and bring to the boil. Whisk in the soy and wasabi then set aside to cool.
Put the radishes and carrot in a large bowl and pour over the cooled liquid.
Leave for 30 minutes, turning occasionally. Drain before using.
Mirin: see Glossary page 138
Julienne: see Kitchen Notes page 141