Place the prunes, vinegar, bay leaf, star anise and 80 ml (2½ fl oz/⅓ cup) water in a saucepan and bring to the boil. Reduce the heat to low and simmer for 15 minutes. Remove the bay leaf and star anise and transfer to an upright blender. Add the warmed veal jus and blend until smooth. If needed, add more veal jus to thin the sauce. Pass through a fine sieve. Return to the pan and reheat ready to serve.