Mark,
I am a ham fisted idiot and, apart from putting the tip-over switch on up-side-down, I managed to change the undertray easily.
I put the new undertray (2nd hand on eBay) beside the bike and when I took a screw out of the old one put it in the new one so I knew where it went. After I got the old tray out I swapped the screws over and as Haynes says 'fitting is the reversal of removal'.
With a trip for a new number plate (and look at bikes) and a panic because of the tip over switch, I managed it in an afternoon.
Would be cheaper if you can find an undertray on ebay.