my sis owns the 2010 non-automatic Fiesta zetec 1.4 and reckons her display says @430 miles to a full tank! I also have a 2013 corsa 1.4 and that says @430 miles to a full tank too - why is my fiesta automatic only giving me @280 - 290?

Being automatic loses a good chunk of MPG.  Have you actually checked it properly?  The computer may not be particularly accurate if you've recently bought the car.

The only way to be sure is to fill the tank and note the mileage.   When you start to run out fill the tank again and again note the mileage.   Take the 1st mileage from the 2nd and divide by the number of gallons you put in on the second filling.   That will give you an exact mpg.

Automatic and E10 fuel be my guess.

 

It will to a certain extent use recent mpg to calculate this.  So if the last 500 miles were driven around town it will predict the miles from the fill up as being lower than if the last 500 miles had been driven along a flat road at a constant 55 mph