The meaning of 'public authority' and Article 5 - just a lawyer's puzzle?

Recently I have been wrestling with a particularly knotty legal puzzle concerning the relationship between the Mental Capacity Act 2005 (MCA) and the meaning of ‘public authority’ under s6 Human Rights Act 1998 (HRA).  The issue is that the protection afforded by the MCA is essentially parasitic on existing law; thus where there are gaps in the protection of the existing law, there will be gaps in the protection potentially offered by the MCA.  This is because the Act does not create any new rights, torts or offences (bar the s44 criminal offence of wilful ill-treatment or neglect), but codifies a ‘general defence’ that protects carers and professionals against liability or prosecution for acts in connection with care and treatment of people who lack capacity.