I don't have the leaflet to hand, so I can't quote the claimed savings. A quick calculation, assuming avg 45.4mpg, which is 10m/ltr, and fuel at £1.30/ltr, equals 13p/mile in fuel.
If the treatment reduces fuel consumption by 10%, then 45.4mpg is now 49.94mpg, which is 11m/ltr, and cost per mile is now 11.8p. That is a saving of 1.2p/mile. It will take over 8300 miles to recoup the £99 outlay.
That is less than our annual mileage, so it would pay for itself in under a year, and assuming 10000 miles annually, would knock about £20 off the annual fuel bill.
But cycling to work an extra day a week would save me ~£1.50 a time.
I've only got to cycle to work an extra 12 or 13 days a year, and I'd make the same saving.
So all in all, saving £20 on an outlay of £99 doesn't seem very good value for money.