What was the car they drove?

I don't know exactly what the make of the car was, but if you watch the DVD with the director's commentary, they mention the car. They said they took an existing car, and then modified it by adding the extra seats at the back so that the entire group (Maggie Smith, Chris Cooper, Timothy Spall, Emmy Clarke, etc...) could ride in it when they took their trip to Sienna. So, the car is not exactly a 'real' car in the truest sense of the word.

I think it's more likely to be a 1947 6C 2500 Cabriolet. The design is reminiscent of the pre-war cars - by the early 50's The Alfas had developed more 'shark' like noses whereas the car in the film has a rounded, laid back shape:

http://www.conceptcarz.com/vehicle/default.aspx?carID=8072&l=A&optionID=5

http://www.sportscarmarket.com/profiles/2005/November/Alfa%20Romeo/index.html

This car is definitely an Alfa Romeo (note it says Alfa Romeo on the radio cover in the center of the dashboard, which -- just like my old 1969 MGB -- would have been fitted when the car didn't have a radio installed).

This car's lines are very similar to the smaller English Austin Healy (with a bit of 1950s Jaguar about it in the split windshield), and the steering wheel with the metal rods radiating out from the center to the plastic wheel is similar to the steering wheel on my 1969 MGB (the standard steering wheel for the MGB from 1962 to 1969).

The specific model in this film is the Alfa Romeo 6C 2500 Cabriolet SS (SS meaning "Super Sport") made from 1948-1952. If you do a Google image search for:

"Alfa Romeo" +6C2500 +Cabriolet

or

"Alfa Romeo" +6C2500

also

"Alfa Romeo" +"6C 2500"

you'll see plenty of images -- even scale model versions.

Note the different front grill treatments above the bumper (not the main Alfa-style grille but below that). The car in the film has horizontal grilles and it appears the 1950 version has a row of vertical grilles here, so the one in the film appears to be a 1948 or possibly 1949 version.

For the scene where they went to Sienna they (according to the commentary track) did a mock "dickey seat" by removing the trunk lid and mocking up a small seat for two of the characters to sit in, so that all the characters could fit in the car.

I also quite like the color of the car in the film -- so pretty against the beautiful Tuscan scenery (passing for Umbrian scenery) in this film.