Yes, only small parts of Fish were radio-based, and NONE of Mostly Harmless. Actually, LTUAE was very different as well; actually, so was RATEOTU... and even the original was quite different... like I say, he re-did the storyline.
The Radio show had a much better way for them to be rescued from prehistoric Earth.
The introduction begins like this:
"Space," it says, "is big. Really big. you won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemest, but that's just peanuts to space. Listen..." and so on.
After a while the style settels down a bit and it begins to tell you things you really nead to know...