PPs mentioned Cath's party appearing 'normal' in the flashback, regarding men/ women ratio, and even a child (children?) dancing. In ep 1, Dim Taxi Man also claimed to have driven Cath's Aunt and Grandmother (some older relatives, anyway...I'm a bit hazy on detail). All would reinforce our image of a nice, cosy, regular 50th birthday celebration.
But that doesn't necessarily rule out darker stuff (sharing of porn/ even amateur filming of) happening alongside, within such a huge house and grounds.
Perhaps kind of a Masons-type scenario, shrouded in secrecy, where being allowed levels of access and knowledge depends on how far you personally get involved, thus unable to whistleblow or incriminate others, if discovered?
Possibly the scriptwriters are hammering home the message that surface 'respectability', sexual violence and a hidden world of porn sharing (or swinging/ adultery, at the very least) can co-exist anywhere, even sleepy seaside towns?
Sidenote: Creepy Convicted Rapist was bloody foolish to intimidate DC Harford. Wouldn't that jeopardise his probation big time? Unless he has some sort of connection to her/ her Dad?