Incidentally Malcolm, who do you rent with? Have only used Avis this far as we get a pretty good deal with them...
i book whoever is cheapest of the big guys via sites like holidayautos, rentalcars, travelocity etc. However, I do avoid AVIS wherever possible. For whatever reason they always have huge queues and long waits for cars (Vegas, Phoenix and LAX in particular). I had a slow puncture on one AVIS car this trip, and went to the 3 nearest AVIS sites for an exchange. None had any cars. In the end I had to go to the hell hole known as Phoenix Airport, and 3 hours later I eventually left with an exchange car.
One of the tricks with lo-cost airlines is to NOT book with their own 'preferred' company. You get off the plane with 200 other PAX, 95% of them renting a car at some portacabin of an airport nowhere near the city it's named after (Milan Bergamo for instance), and the queue for their preferred partner is out the door and round the corner. Book with one of the other suppliers at the airport and you walk straight up to the desk.
Met some Dutch guys this trip - they'd flown into Atlanta, got to the car hire desk to pick up their pre-paid rental, and AVIS had no cars. They waited 4 hours, and still no cars so booked a local hotel and came back the next morning. Still no cars, except an electric VW UP I think it was. They had little choice, but still got charged extra for the upgrade! Met them at Savannah Georgia (excercise William Tell), and we both left around 3pm for the trip up to Virginia Beach (Oceana Air show) - about a 7-8 hour drive. I was in bed in the hotel by 22:30. They arrived at the hotel between 02:30 and 03:00, after 3 different 1 hour charging stops and slipstreaming lorries at 60mph for the whole trip.
And remember that AVIS=Budget, Hertz=Thrifty=Dollar and Enterprise=Alamo=National. If AVIS haven't got any cars, then Budget won't have any either.