Send them a letter by recorded delivery stating your reasons why you consider that you were unfairly forced into taking out the PPI which you most definitely did not want.
Ask them for the full return of your premiums paid plus interest if any, inform them you expect their agreement to your request, otherwise after eight weeks (if still unresolved).... you will take your complaint over their unfair misselling practice to the financial ombudsman... as per your legal entitlement.
http://www.financial-ombudsman.org.uk/consumer/complaints.htm