After I stopped chuckling at Treworgey's comment I thought I would add my experience.
I have a 2002 TD6 and wanted to upgrade the standard 18" alloys. I read several different articles on internet of the experiences of those that had changed to 20" plus alloys encountering problems with vibration through the steering. My local dealer confirmed it could be a problem. However seeing that the latest diesel Range Rovers fit 19" alloys, I bought myself a set of 19" and have not encountered any such issues as yet.