Thanks for all the help guys. After a bit of research I discovered that as well as the R1200RS the 2015 F800R shares the same front mudguard as the R1200R LC. A quick trawl of ebay revealed a matching F800R white mudguard in a breakers in the UK for £69 ($89) including postage. Took a punt and ordered it yesterday, it arrived today, it is a perfect match, in superb condition and is now fitted.
Very lucky indeed, I will not be fitting a fender extender.