I finally tracked down a problem to the WAP plugin that affects Confluence 2.9.1, if enabled this plugins presence causes Confluence to fail to deploy. Once in this state Im dead and have to restore from backup unless I can figure out how to unpick a plugin from a live system... On enablement of the plugin I get :

Transaction rolled back because it has been marked as rollback-only

The logs contain references to NoClassDefFoundError: com/atlassain/confluence/core/actions/NotFoundAction not sure where this class comes from. the WAP plugin lists compatibility with Confluence 2.10something but there is a problem here for 2.9.1