The first thing to consider when making a complaint about dentistry is whether the work was conducted on the NHS, or paid for privately. This alters the route you should go down when making a complaint.
If your treatment is NHS funded, then the complaint system is very similar to the complaints system for a complaint against a GP.
You should first write to the manager of the dental practice, setting out your concerns. If you are not happy with their response you can contact your PCT. You can find out the details of your PCT at www.nhs.uk.
In the same way as NHS treatment, your first step in making a complaint about private treatment should be to write directly to the Dental Practice that carried out the treatment.
If you are not satisfied with the response you receive from the practice, you should complain to the Dental Complaints Service. This is a body set up by the General Dental Council to hear complaints about private dentists. Their website is www.dentalcomplaints.org.uk and their phone number is 08456 120 540.