Hi Charlotte,
I agree with Harters, leave it poss until start of 2009. However, I do have some other advice.
Doyouspain are very reasonable (but use some companies that have full to empty policy) BUT if they say special offer on some models it usually is with Record who operate a return car with same amount of fuel as when you took it - it is all recorded properly - so you only pay for the fuel that you use. You can check out Record directly but they are generally a bit more than going through Doyouspain.
Might be worth trying DYS & in the booking process in the request/comments box state that you want an on site company that you will only pay for the fuel you use. Remember not to pay up front (pay at pick up option) & you then have the option to change to another company without losing out if the deal they come up with doesn't suit.
Hope this helps