Dishonorable Business Conduct

I bought an NP200 from Citton Cars in January 2019. At the time, it had a defect which they were aware of but unable to sort out, despite the vehicle going back more than once for up to a week at a time. After I obtained a defect report and quote from BB Nissan Hatfield, Citton Cars opted to replace the steering rack themselves, but said the vehicle had to stay with them for a week in order to do so. Since I use the vehicle for my business, I had to wait until 2 May 2019 to send it in for the replacement. Today, after having had the vehicle for more than a week and me having to repeatedly follow up with them, they said they will not replace the steering rack at their cost, "because the vehicle had been driven 15,000 km" since then. This is nothing but dishonorable business conduct, and I will not rest until Citton Cars does the right thing and replace the part at their cost.