Renault - Faulty Gearbox

I purchased a Renault Clio RS in 2012. In 2013, I realised that there was a problem with the Gearbox as I could not select the 3rd Gear. I immediately contacted Renault Bryanston and made a booking to have this investigated. It is noteworthy to mention that the vehicle is still under warranty. They booked the vehicle in and after extensive investigation and 6 months later, they decided to replace the Gearbox with a new one. Soon after I received the vehicle (6 months) , the same problem occurred at which point, I contacted Renault Bryanston yet again. Needless to say, the vehicle has been with them for over 2 months now, where they are apparently awaiting approval to replace the Gearbox a second time. (They are unable to find the fault) I wish to log a formal complaint against Renault SA for selling me an inferior product as well as Renault Bryanston, for lack of communication and focus. All I need is to have my car back in perfect working condition......before the warranty expires.