It's true there are stacks of hurdles to get over even if someone does buy an option. But I bet independent UK film companies are a lot more author-friendly than the big US ones. Ruby Films went out of their way to involve me in the tv adaptation of my book.
Yes, it obviously varies hugely how much input you get.
I can get my head round someone buying an option and then not being able to raise the money to develop the script, or then to make the actually movie/series, but the depressing bit seemed to me when they buy the option purely as to prevent someone else making a film out of your book which might spoil their movie.
Mind you, the headline screenwriter isn't always the one whose words are left in the script by the time it's shot. I remember a Front Row piece about it, a propos the recent P&P.
Presumably she makes sure she has the copyright on it, and can then do what she likes. I imagine the IoS isn't too bothered about the circulation of the others.
Clive Barker also complained that once the film rights are sold, the film can enter production and still never been made. We (and he) have been waiting for a Weaveworld movie for over over ten years).
I'll agree with The Black Dahlia being a turkey. Loved the book, the film just stunk like the corpse it was based around. It's one redeeming feature was that it had Josh Hartnett's bum in it.