I have the radio play on cd and the tv series on vid and the books on paper and although there are, as you say, deviations between the 3 they do at least follow the same plot and story line. It seems to me that the storyline for the film was completely rewritten. I know DA had a hand in writing it but I calm myself by insisting that the other writers must have changed it after he died. If not, then DA should (if he were still alive) be horsewhipped for allowing and even contributing to such a bastardisation of his work.
The only good thing I found in the film (and even that was different) was the way they did the two heads of Zaphod. And then they went and cut one off.....???????
Anyone want to buy a slightly used copy of HHGTTG?