You will find that your insurance company will have written on your policy document exactly how you can make a complaint, if you need to and who to contact. Before you contact an outside body, you can take the matter higher within the insurance company concerned. Most companies have a Customer Services Section at their Head Office who may be able to help you. If not, try contacting (by letter) the Chief Executive of the company.
All organisations selling insurance are bound by the Association of British Insurer's General Insurance Selling Code. If you are unhappy about the way an insurance policy has been sold to you contact the insurance company who underwrites it to complain.
Your broker may also be authorised and regulated by the Financial Services Authority (FSA). If so, further information about complaints can be found on the FSA Website