Just to add as part of seeing this thread originally I tested my 2017 420 R SV Diff at 3500K mls (car purchased from Caterham used in summer 2020) and the LSD was knackered no backlash at all with the tests, so I bought a non LSD diff and got Tracsport put his LSD in which was then fitted by Nick Potter, this has been brilliant ever since. As has been said above the RBTB were about the same cost I guess I also spent more to resolve this at roughly £ 3400 for new diff, Tracsport and fitting. Not sure if this is useful or not. PS if anybody wants to go down the same route I am happy to work out a deal with my old diff which is compete and checked but just has knackered carbon plates.