Part Number RGU100531 is NLA according to Xpart/Rimmer.
I suspect the only way you'll get one is from a scrapper/dismantler.
Maybe try a forum second hand parts trader?
This one on eBay looks about as new as you'll get:
http://www.ebay.co.uk/itm/Rover-75-M...AAAOSwAuZX2quF
TC