Complex sentence: It was an absolutely miserable day in Edinburgh which I imagine might have prevented some people from attending but about fifteen people turned up some of whom had never edited before, and new articles were created and existing articles expanded, tidied up and improved.
Split into simpler sentences: It was an absolutely miserable day in Edinburgh which I imagine might have prevented some people from attending. Still, about fifteen people turned up, some of whom had never edited before; by the end of the day new articles were created and existing articles expanded, tidied up and improved.