Just to address the marmite topic brought up by another reviewer: It was illustrated in the film that Billy Nighy's character Alan was not a reliable story teller. He liked to tell tall tales, like the one about the grandmother with the coal seam in the basement.
Going back to the movie, I thought it was quirky like everyone mentioned. I knew it wasn't going to be a straight forward dramedy based on the bizarre, artificial riding in car scenes. It looked like it was done in a 1950s B movie style.
Thankfully the movie dials back on that and delivers a fairly straight forward, drama comedy framed around the game of scrabble, but really about Alan and his son Peter, as they try to solve the mystery of the missing son Michael. The body viewing scene (which we don't see) was particularly odd, because we were told that Michael went missing many many years ago when he and Peter were still children. So why would they be called to identify a body? If it was an adult's body, how would they know it was him. Usually they do DNA tests anyway.
Other than that weird start, and some scene involving a boat (or did it?), much of the movie follows a fairly linear format. I think it helps that every character in the movie had something to offer the movie, and generally everyone was likeable. You kind of hoped there would be a resolution of the 'mystery', but ultimately it didn't really matter. Nothing is really 'solved' by the ending, except maybe Peter wasn't as resentful of his missing brother's 'Prodigal Son' stature, and maybe Alan realized it was time to focus on his existing family.