The best car hire deals are ALWAYS through specialist "broker" companies offering better rates than the actual hire companies. You get quotes and book over the Internet or by telephone, and you must book several days in advance. They are always a lot cheaper than dealing direct with the hire companies. It's worth checking ALL the company web-sites listed below, because they vary greatly; by country, time of year, pick-up, etc. It is not safe to rely upon a company simply because you have used it before!
PLEASE CHECK THAT YOUR INSURANCE COVERS YOU FOR BOTH FRANCE AND SPAIN!! Increasingly, people are discovering that companies at Gerona in particular are charging extra for insurance cover in France, but this is not always made clear. It seems that car hire in Barcelona usually does not include driving in France. Also watch out for companies that make you pay in advance for a full tank of fuel and say you should return it empty - which is of course nigh on impossible!
Over the years, a good company has been NovaCar (Freephone: 0800 018 6682). In 2001, NovaCar offered a Group A (Ford Fiesta 3 door, no Air Conditioning) for about £130 sterling for 7 days, fully inclusive, including airport pickup. You need to book 4 days or so before you travel! In early 2004 they were far the best of the bunch for Italy.
However, in October 2003 I used World Wide Reservations. I hired a Ford Ka for 6 days; I picked up the car at Stansted airport and dropped it off at Birmingham airport for a total of £111 compared to the £154 quoted by Nova!!!! The car hire company (they both specified National/Alamo) and the type of car was the same in each case.
I have also found a web site for Auto Europe, who seem to offer a very good price (eg under £90 for a car from Gerona airport for 7 days including second driver).
Try also Holiday Autos.
Failing these, there are three or four companies who have booths at Perpignan airport, and they always seem to be manned when UK flights arrive. Budget Car Hire are one such company. Tel: (0033)(0)4 68 56 95 95 ; Fax: (0033)(0)4 68 85 18 24.
Ryanair has a deal with Hertz , and in 2008 we have found that they are often the cheapest, at least in the UK.
Carcassone airport is very small and there are no permanaently manned car hire desks, so beware - you must book in advance!
For Barcelona I have used a UK company called Flightline. They use normal hire companies but can save you up to 50%. The UK phone number is 0800 0360 777.
Gerona is 60 miles closer to us than Barcelona, and car hire is available at the airport.