I'm with you on the understanding of reason for Patrick's arrest. I thought that was very clear in the book and they kept that in the film, didn't they? I only watched it once so far and without subtitles,and didn't get all of what Tom said to Marion about it but he said something about the alley behind the gay bar. Also the timing would seem a bit off for him to be immediately arrested based just on the letter. (I got the feeling at least some people in the museum, like the woman who informs Patrick of Tom's first visit, knew or had suspicions.) And it does make it worse because it shows the threat of being found out is everywhere and pointedly also with other queer people who are being pressured by the police. Bethan Roberts made a great choice to write it that way. The personal decisions of individuals do have an impact (Marion's guilt afterwards etc) but it's the social circumstances and especially the state violence that are the biggest villain, so to speak. I actually found that an important point in making the historical setting work in the book. One of the biggest wave of trials against hundreds of queer men in Germany after WWII (in Frankfurt in 1950/51) was triggered and based on the arrest of one guy they questioned over and over. That's what I thought of when I read the book.
Really glad to hear other people understood the book as I did, because I've seen a lot of people talk as if Marion's letter caused what followed and that's not how I understood it at all. It's hard to be definitive about anything, because the whole point of the book is that there are two unreliable narrators.
To me Patrick's explanation - that the man he'd met at the Argyle had got into trouble and grassed him up made perfect sense and didn't need any further explanation.
Whereas the idea that Marion sending a message to the Head of the Museum (who Patrick knows is not straight) would have ever got to the police, let alone the police then being able to reverse target Patrick through this guy, was incredibly farfetched. Sylvie explains this to Marion and I found her logic entirely convincing.
I think the changes they made - having Tom say that someone had given them up and it must have been someone at the museum, the cutting of the Sylvie scene and the slightly different way Patrick talked about it, were all about presenting a cause and effect that just isn't there in the book (and doesn't really make sense).
I do think there are really good pacing and story structure reasons why they'd make this decision in the film. On top of that hte film is all told from Marion's point of view, in a way the book isn't. So it sort of becomes a plot hole, rather than a point of ambiguity.











