In the first instance you should complain directly to your solicitor about the poor service you have received. All solicitors have a procedure for handling complaints.
Each solicitors firm will have a designated complaints handler whose details may be obtained from the Solicitors Regulation Authority Contact Centre. Tel: 0370 606 2555 or email: email@example.com
If you've complained to your solicitor about poor service or about their bill, and you aren't satisfied with your solicitor's response, you should contact the Legal Ombudsman on 0300 555 0333, who can help to resolve your complaint for you.
How to complain about legal professionals who work for people with mental health problems
The Law Society has developed guidance to assist those with mental health difficulties and those working in mental health to complain about the service or conduct of a solicitor or other legal representative.