Unwilling to claim responsibility, poor customer care

I bought my Renault Sandero (second hand) still on service plan in 2015. Before purchase it was serviced by a Renault dealership, but shortly after receiving the vehicle I experienced issues with the gearbox, and after taking it to Renault (The Glen) was told that a bolt holding the gearbox in place had been inserted incorrectly and had broken off. This was repaired by Renault with no cost to me. Thereafter, I continued servicing my car through Renault, with the last service in July 2017 being done at The Glen. In January 2018 I noticed a strange "clicking" sound when driving and decided to take my car for a service (as it was due) and queried what the clicking" may be. The (private but qualified) mechanic whom I handed my car to (my car is now out of service plan and Renault is exorbitantly priced, hence my decision to use a private mechanic) called me to say that upon inspection, it had been found that the spark plug had been inserted incorrectly and had been STRIPPED and therefore required replacing (i.e.: a new helicoil). I contacted Renault The Glen and spoke to Chantell and Jerome (service manager) who addressed my concerns poorly. I was told that since a third party (private mechanic) was involved, that the technicians at The Glen would need to see the car first in order to decide whether they are at fault. When I told them this would require towing my car since it had only 3 out of 4 spark plugs, I was told that they would not assist me and that this would need to come out of my own pocket. They also claimed that their technicians would not make mistakes like stripping the plug, etc, etc. They also referred to my chosen mechanic as a backyard mechanic (which I do believe iss SLANDER, as he is a qualified service provider). I feel very strongly that I received exceptionally poor responses to my concerns and will definitely NEVER be returning to Renault for ANY service EVER. I will be fixing my car out of my own pocket and will then get rid of the vehicle as soon as I possibly can as I do not want to be affiliated with a brand that has no regard for customers at all.