I retain some skepticism as to how much actual writing on this was done by Mr. Cleese...especially given his second-place billing in the credits. I can well imagine (what follows is pure speculation on my part) the pair idly sitting around, riffing on the idea of a British-raised Superman, Johnson perhaps noting ideas by Cleese and incorporating them into the story. And when it came to writing the credits, a publisher would be a fool not to recognize the potential sales boost in being able to put the name "John Cleese" on the cover.
The above should not be taken as any slight to either Kim Johnson or John Cleese, both of whom I think are swell people. But I've always thought there was a bit of hype in this...