The usual suspects Ghostery and Ublock Origin weren't the issue (or resetting the modem many times), and I see over on the Twitch subbreddit a number of Spark customers are having similar issues, and also some Bigpipe customers. (Over vodafone and 2degrees mobile its going fine). Vods and all other internet traffic seems fine - just twitch live broadcasts.
Anyone else (other providers) having this issue?