Your first step should be to make sure you have as much evidence as possible to back up your claim. Keep as much paperwork as possible and make notes of what was said in any phone calls or meetings.
You should also make a written complaint to the adviser who you think has been negligent. They should then acknowledge your complaint, explaining their complaints procedure and how quickly you can expect a full reply.
If you have any concerns about the best way to make a complaint, or are unhappy at any stage with the way the adviser is responding, you should take legal advice. A free initial enquiry will help you understand how strong your case is and what your next steps should be.