I was expecting the ending to have a real twist and was imagining all sorts of convoluted scenarios. But I think it was better the way it was.
I did enjoy it very much, even though I disliked most of the characters. I thought the two daughters were mixed up and badly behaved but I didn't think they were horrible. But the son was a nasty piece of work.
I think Mark was actually quite a weak person - people who need to control those close to them and who feel threatened if they demonstrate feelings for anyone else are, I think, underneath it all insecure themselves. My view is he was dosing himself up with Diazepan because he was depressed and worried about the party. He smoked a cigarette and drank too much in an effort to deal with his anxieties. His true unstable character was exposed when he resorted to swearing at the family and being generally obnoxious.
I agree with suziewoozie re Vivien. I think she was really annoying and, like Mark, immature She was also, in her own way, quite controlling herself, judging by the look she gave Mark when she found him smoking.
There were a lot of loose ends which left me puzzled, but I'm not sure if it would be a good idea to have another series.