This is my complaint against a arnold clark branch, as a paying customer what would you expect as the response?

You stated that you change your car every 3 Years, therefore the problem you had should be repaired under warranty, I don't know where you live, but I would phone my local paper with this story and have it plastered allover the front page, you can also get it touch with the "Customer Service Department" (phone number in your local phone book) they will sort it out for you, alternatively you can see a solicitor, apart from that your letter is spot on.