Having heard the "transcript" that the neighbor recorded, it sounded to me like Boris had decided to stay up late probably researching for his hustings the next day, while the girlfriend was feeling miffed at the lack of his presence in bed. So she grabbed his laptop from him, he was heard saying "get off my laptop", he went to wrestle it back from her, she was heard yelling "get off me" and in the tussle some wine spilt on her sofa, and it all escalated. Although I hate Boris, I think this argument was largely the woman's doing.
HOWEVER... what worries me is, this woman doesn't sound like the typical prime minister's partner who usually kind of put up with the fact the role in office is going to come very much first for a few years and tend to remain in a quietly supportive role. How is he going to function at his best with a woman who is making relationship demands on him that conflict with him needing to spend long into the night working, like pretty much all PMs have been known to?