Sorry for the delay. My client had difficulty in arranging downtime.
As suggested we also raised a SR with VMware who advised the same thing. However on the initial attempt to update following the changes to the database tables, the update failed. I reverted to the old version and SQL backup. Updated the schema, but this time I uninstalled App Volumes 2.16 and installed 2.18, but I also changed the database account to a SQL one rather than the Windows account that was originally specified. The update was successful.