Just stumbled upon this video and now I know what the little white (unused) plastic funnel in a bag under my boot floor was meant for! I'd previously guessed it was maybe for topping up screenwash although it's not got much of of a funnel shape to it!
Mystery over! Odd that I've never needed to use this to fill up so why did they provide it I wonder?
https://www.youtube.com/watch?v=gg2mEfDarC0