Shocking service from both Insurance, Repairer and Assessors

I am writing to formally express my dissatisfaction with the level of service I have received from you. As a customer who has maintained a policy with your company for a while, I am deeply disappointed by the recent experiences I have had and feel compelled to bring them to your attention. Once again, 2 months after my accident, I have had no word from Discovery as to the progress of repairs on my vehicle. Finally today, I am called by the repairer to be told that the vehicle is ready, however the front headlight which was replaced (used part) is not correct and a new headlight needs to be ordered. On asking why this wasn't done in the first place I am told that Discovery authorised only a second hand part. Now I need to wait a further week (9 weeks in total) for my vehicle to be ready. I need Discovery to give the repairer authorisation for a new part immediately. Angry, furious and frustrated client. If I do not receive a satisfactory response today, I will be left with no choice but to consider further steps, including lodging a complaint with the relevant regulatory authorities.