Add the chillies to a saucepan of boiling water and cook for 2–3 minutes until just softened. Tip into a colander, rinse with cold water and drain well.
Pour the vinegar into the drained pan and add all the remaining ingredients. Heat gently until the sugar has dissolved, then cook over a medium heat for 5 minutes.
Pack the chillies and herbs from the vinegar tightly into 2 jars, then pour over the hot vinegar mixture, making sure that the chillies are completely covered by the vinegar. Cover with screw-top lids, label and leave to mature in a cool, dark place for 3–4 weeks.