You should first complain directly to your solicitor by writing to the designated complaints handler at the firm.
If you have already done so and are not satisfied with their response you need to contact the Legal Ombudsman. Their contact details are:
Telephone: 0300 555 0333 (09.00–17.00)
PO Box 6806
The Legal Ombudsman is the independent ombudsman for all legal services and complaints about your solicitor need to be made directly to them.
If they are not your solicitor you need to report them to us, the Solicitors Regulation Authority. To do this, please complete a proforma (DOC 4 pages, 122K).