Clever, fun, and well-made I'm puzzled by the relatively low score for this film on IMDb. Anyone who was expecting a fast-paced blockbuster 'whodunnit' obviously didn't do their homework beforehand.
Yes, it takes a while to warm up, but it's never boring (unless you're lacking in intelligence) and is great fun to watch. The humour varies between subtle, dark and slapstick/farce, but there's a subtext of emotion and tragedy in the main character's back stories.
Okay, so the plot is a little predictable at times, but the fun comes in the form of the 'real' people, like Dickie (Richard, not David!) Attenborough and Agatha Christie etc, and the interaction between Constable Stalker and Inspector Stoppard.
It's not meant to be taken too seriously, it's just an hour and a half of old-fashioned fun. And Saoirse Ronan steals the show with her portrayal of an enthusiastic but inexperienced police constable.
The direction is fairly slick, with 'Hot Fuzz' style cut scenes and maybe the split-screen was overused a touch, but for me it didn't detract from the enjoyment. There's also a lot of flashbacks, and this method is even commented on by one of the characters!
In the end, this is self-parody at its best, a gentle poke at the classic whodunnit that we know and love, with all its failings, plot holes and repeated themes.
So, ignore the haters and indulge yourself!