Unfortunately there was no price breakdown in the app, just a total. It seems that the difference was due to the fuel option. Having completed the rental I am happy to say I was not charged for any fuel.

However, couple of other things at the counter didn't line up properly with the prepaid reservation voucher (eg the location surcharge), so it remains to be seen what they will charge.

Also for some reason they created a new reservation (with a new number) and transferred across the voucher from the reservation I made. Fingers crossed that this will not mess up my getting 9000 Avios in the excellent Iberia promotion.