Pour the vinegar and sugar into a saucepan, add 4 each of the rosemary and thyme sprigs and bay leaves, all the lavender, if using, the garlic, salt and peppercorns. Cook gently, stirring once or twice, until the sugar has dissolved. Bring to the boil, then boil for 3 minutes until the mixture becomes syrupy.
Pack the plums tightly into warm, dry jars and tuck the remaining fresh herbs into them. Strain in the hot vinegar, making sure that the plums are completely covered, then top with airtight lids.
Label and leave to mature in a cool, dark place for 3–4 weeks. The plums will lose colour slightly.