I think a lot of Martin's bad behaviour stems from Arthur dying as he was only ten/eleven when it happened. He had a better relationship with him than with Pauline as she just always got on his case and nagged and shouted at him whereas Arthur was more laid back with him.
Why didn’t Martin say to Mark that he wasn’t his Dad so he couldn’t force him to eat. I don’t like parents forcing their kids to eat let alone their controlling siblings! 🙄