Follow the company’s complaints procedure. You can usually then take your complaint to the Financial Ombudsman Service if you are unhappy with their response.
Ask for a copy of the company’s complaints procedure. This should tell you how to complain and how they will deal with it.
If you are unhappy with their response (or they do not respond within 8 weeks) you can complain to an independent complaints service. This is usually the Financial Ombudsman Service - the company’s complaints procedure should tell you which service to contact.
The Money Advice Service (external link) has information on making complaints and time limits for complaining to the ombudsman.