I had this cap fitted by Steve to my Series I and had the same experience. Easi-fill.
My current car goes to Steve in November for some work and included is the fitting of a Monza cap, as I think that the Yamaha cap is even worse for filling than the one on the Series I and as BtG says, quite often ends up with petrol over the boot.
Enots - Monza Caps from Think Automotive 
As you can see, the Aero cap is also marketed by the same people and may be more appropriate for an Aero 8 as it draws its inspiration from a fuel cap used on American world war two fighter aircraft.
