Thanks for that.
So it seems Writs appear about 3% of the time, which seems like a lot of slog, since Common Medals, Delve Rewards, Faction Hoard components and Pets which I already have maxed out etc aren’t of much use to me 
I might try some more Arena battles, but I’m getting back to the “it’s not worth the effort” feeling.
Has anyone actually compiled a list of Arena rewards and their frequency?