I must offer my thanks to Geoff Brown and others on this thread who brought this issue to my attention. I bought my 2015, 420R in June from Sytner in Oldbury. They took it in as part exchange on a Porsche Spyder and sold it to me with a 6 month warranty. Having under 8000 miles on the clock I wasn't expecting any issues, but having read some of the posts and after carrying out a wheel rotation and preload test, was concerned that my diff was suspect. I contacted Road and Race and spoke to Ollie for some advice. He recommended speaking to DPR Motorsport who were happy to deal with the warranty company and manage the repair. The diff was taken to Road and Race who found that all the carbon had fallen off the plates, the Belleville washers were shattered, the bearings were gone and the carbon had attacked the surface of the CWP. They supplied a complete new diff and DPR put it all back together, using Road and Race's recommended oil. David and Daniel at DPR couldn't have been more helpful and got everything sorted under the warranty, Since collecting the car I have noticed much improved traction and a more predictable rear end, it is actually very quite with no undue noise or vibration. Very pleased that I was alerted to this while I still had a warranty as this was not a cheap exercise, but hopefully it will be good for a few years now.