Rental prices on the website are exclusive of fuel costs. You must therefore return your vehicle with the same amount of fuel as it had at the start of the rental.
If the gas level is inferior at the end of the rental, the owner can proceed to an adjustment in order to compensate the difference.
If the gas level is superior at the end of the rental, you can proceed to an adjustment yourself, if the owner didn't take care of it for you.
NB: For Open rentals, a 15€/£ penalty will be applied if you give the car back with less fuel than at the start (if the difference is 15% or more of the tank capacity).