Plot hole: When Daisy is killed, Dan comes around to collect her things. So we'll be able to see him reading one of Daisy's love letters from Ben and accuse Ben of Daisy's murder, when Dan takes the box from Stuart, who is holding it with his hands underneath, keeping everything in, he holds the box with his hands on the side, deliberately, so the love letters fall out to the ground - and how convenient that nothing else falls out, just the love letters. But the letters were written before the girls arrived - they saw them - so how could Dan accuse Ben when the blue one has Ben asking Daisy to meet him in the summer house, but it was days ago, not relevant now.
Plot hole: Dan, the policeman, finds a love letter written by Ben to Daisy, Dan's sister. The letter says "Meet me in the summer house tonight." When the girls arrived 2 to 3 days ago, Ben and his brother were arguing about the letter, which was written on blue paper. When Daisy is killed, Dan, ridiculously, accuses Ben - only on the blue letter as evidence, which we know was written days ago, and Daisy was killed in the afternoon, not at night as the letter says; plus, Dan says Daisy was struck with a heating tube, just like the ones where Ben works on the winery, but anyone could have picked it up. The evidence is stupid, but Ben is arrested, placed in handcuffs and taken away.