Hammer purchased the rights to Brat Farrar in 1958 and the film kept appearing on its production schedules for a while. Paul Dehn (a film critic for a national newspaper) was hired to write the script. He wrote at least a treatment back then which, based on a Hammer press release, seemed to be much more faithful to the original novel and included the show jumping background.
The project was then shelved for a few years. It then resurfaced in 1962 when Hammer decided that psychological thrillers were back in. Jimmy Sangster wrote a new script from scratch which, as many have noted, differs a lot from the novel. One would guess that most of the differences were due to budget - hence the whole show jumping stuff was scratched as too expensive and the action confined to a single location using only 4-5 main characters.
I had wondered why Tey's novel wasn't credited at all.

great dramatization of the novel.

I wouldn't judge a film simply by how faithful it was to its source material. A film can have its own identity, and I honestly thought the Hammer production was a more impressive piece of film-making than the 80s adaption you mentioned.
Just down to taste I guess.

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If Josephine Tey did request her name be removed from the credits of Paranoiac she must have done this via a medium.
She died in 1952.

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PARANOIAC has enough similarity to BRAT FARRAR that when I first watched the film 3 years or so ago I immediately thought "damned if this isn't similar to BRAT FARRAR by Josephine Tey" - I'd read several of Tey's novels, but didn't see her name in the credits, though a quick check here at IMDB showed the novel as an "uncredited" source - Did Jimmy Sangster actually try to pass this off as an original screenplay, hoping that by then Tey's novel was obscure enough that no one would notice? Or did they attempt to buy screen rights but couldn't afford them? Who knows? Ver-ry inter-resting!
It's a well-done thriller but could have used a lit-tle more character development.
"If I'd been a ranch they'd have named me the 'Bar-None'."