Hi Philippa,
We are currently reviewing our procedures in relation to Customer Service and procedures for difficult customers and have used the ‘Managing unreasonable complainant conduct – a model policy and procedure NSW Ombudsman 2012’.  It is a comprehensive document and provides thorough guidelines.  I am working on condensing and localising for our organisation.
http://www.ombo.nsw.gov.au/news-and-publications/publications/guidelines/state-and-local-government/managing-unreasonable-complainant-conduct-a-model-policy-and-procedure
Kind regards,
Helen