My 172 Cup rear shock went 2 weeks before the warranty ran out @ 3 years old and on 26k. Initially the dealer insisted wear & tear etc. Argued with customer services, got no where. Bought new shocks for MOT.
But customer services made the mistake of telling me that they have no tehnically trained people at all. So i wrote a couple of letters to Renault citing a technically based argument, knowing full well the customer services bods would have no idea wtf i was on about.
Eventually got paid in full for 2 shockers, as a "gesture of goodwill"!!
Just keep on, and on, and on at them.