Replying to my own unanswered query here - after Triumph failed to sort it out at two services, I finally got round to fixing it myself the other day.
The pod came off easier than I thought - two screws on the back, and it splits (careful with the wires that connect the front/back halves).
Wrapped the bar in a single layer of vulcanising rubber tape under where the pod sits, and covered half of the little hole in the bar that nob on the front half seats into with the same (lightly stretched).
Refit the pod - and yay! Finally, no more wobbly indicator controls.
Took me less than 5 minutes - why the Triumph dealership couldn't have done this, I've got no idea.