Nothing done by, or by a member of, a local authority or by any officer of or other person authorised by a local authority shall, if done in good faith for the purpose of executing Part III, subject them or any of them personally to any action, liability, claim or demand whatsoever (other than any liability under section 19 or 20 of the [1982 c. 32.] Local Government Finance Act 1982 (powers of district auditor and court)).