I just re-read this thread and thought about the 1975 publication date for the first book.
So going a bit off piste here, Morse was a chief inspector, he lived in a flat in an old house which I don't think would have been enormously expensive back then, but he indulged himself with the finer things in life. That would mean he was quite well paid, with low outgoings but having to drive around Oxfordshire for work might he not have gone for a new car. Is the old car idea another BBC invention? Also Lewis had to take Morse's car out once and remarked on its' performance.
So here's a guess for the Beta Boys.........a dark blue Beta Sedan. Elegant in a modern way, still quite special in the early seventies before rust (or was it Esther Rantzen and the daily Hate Mail ) tarnished its' reputation and performance worth remarking on.
Now somebody will tell me that the books say it was an old car and spoil my idea.
Frank