Preheat the slow cooker if necessary; see the manufacturer's instructions. Pour the vinegar and sugar into the slow cooker pot, then add 4 sprigs each of the rosemary and thyme and the bay leaves, all the lavender (if used), the garlic cloves, salt and peppercorns. Cover with the lid and cook on high for 2–2½ hours, stirring once or twice.
Warm the 3 clean jars in the bottom of a low oven. Pack the plums tightly into the jars. Tuck the remaining fresh herbs into the jars. Strain in the hot vinegar, making sure that the plums are completely covered, then seal tightly with rubber seals and jar lids.
Label the jars and leave to cool. Transfer to a cool, dark cupboard and store for 3–4 weeks before using. Once opened, store in the refrigerator.