You could remove the recessed bit that the current filler cap sits on, or enlarge the hole in it, which would allow you to centralise the filler cap and the new, longer, hose you would need.
I believe the flap came about due to emission regulations, which forbade vented caps. At that time flip-top caps were not suited to this, and MMC had to use a commercially available cap that was so ugly it had to be hidden behind a flap.
