The Legal Ombudsman (LeO) has published some useful FAQs about handling complaints.
Here is the link: https://www.legalombudsman.org.uk/media/iatjbphd/guidance-scheme-rules-faq-april-2023.pdf
Here are the contents chapter headings:
Chapter 1 – Introduction and definitions
Chapter 2 – Who can complain about what
Chapter 3 – What authorised persons must do
Chapter 4 – When complaints can be referred to us
Chapter 5 – How we deal with complaints
Chapter 6 – Case fees