It is partly possible - I've now found the menu under OBC where you can reset all or individual readings. However, the corresponding menu under Trip Computer only lets you either reset all, or select automatic reset, which presumably also resets all readings, though this option wasn't ticked, and as you can see from the Trip Computer screenshot I posted earlier all the settings apart from mpg were unchanged, which should not be possible.
In any case, I don't think I've ever ventured into those menus, and it's unlikely that I would have accidentally gone into quite a deeply nested menu and scrolled down to just one setting to reset the mpg in the OBC, and so far as the Trip Computer menu is concerned, it seems it is not even possible to only reset mpg there.
So, my conclusion is that there has been some sort of glitch and I will ask the dealer if they are aware of any such problem - not that I expect to get much sense out of them!