Jim Prideaux calls the Alvis irreplaceable. Alvis cars were produced in Britain from 1919-1967. The company was purchased by British Leyland, which itself was partly nationalized in 1975.
Although in the story Toby Esterhase (Bernard Hepton) is presumably junior in age to Bill Haydon (Ian Richardson), in real life Hepton was 10 years Richardson's senior, and had in fact directed the 24-year old Richardson in a critically acclaimed performance of Hamlet in the late 1950's.
In the novel the Alvis owned by Jim Prideaux was likely a 1963 TD21 Saloon, capable of towing a caravan. In the series the car shown is a 1939 4.3L Vanden Plas tourer. Red Triangle revived the Alvis marque in 2012 and currently offer 6 models in limited production.
While talking to Smiley about his encounter with Toby Esterhase, Jim Prideaux (Ian Bannen) says "He (Esterhase) said the Circus had nearly gone under due to Operation Testify and I'm currently the number one leper." Years later, Bannen played a number one leper (a leper king) in Braveheart (1995)