The dealership said they were really surprised that VW approved the warranty claim in less than 24 hours, especially for something of this value!
My suspicion for the failing is that someone has messed up when performing the Haldex oil service @ 40k miles, possibly draining the diff by mistake.
Does anyone know if the new 'Final Drive Unit' would come from VW complete with fluids or would these be left for the dealership to add?