You save very little.
Providing your car can do 41mpg, that equates to 12pence a mile @ £1.10 a litre.
According to their testing, there was a 20% increase in fuel economy, that means it would be 48mpg, now costing 11pence per mile @ £1.15 a litre.
^ That was from "Count-A-Strong" the link he posted, the car was the "Hyundai"
However, looking at the more fuel hungry motors, the sub 30mpg Focus for instant, without the 99oct fuel, it equated to around 17pence per mile, and with, at around 14 pence per mile.
You would have to do your own testing to see if you gain or loose based on your own driving styles.