No idea, I'm afraid, but I did find 'Mostly Harmless' to be a fairly unsatisfying book.

In fact, while I own all the others and both Dirk Gentlys, I do not own a copy of 'Mostly Harmless'.

Have you read 'The Salmon of Doubt'? It's a strange pastiche but the parts of the book that they include do make me wonder what the final book would have been like.