Hello Malcolm,
What is interesting is that you describe the airplane as becoming noticeably unstable in yaw with an aft CG adjustment ... I strongly suspect that you are simply less unstable in yaw now.
Add side area in the tail: and I don't mean a measly 10 square inches per side, but at least 25 squares per side. I had 15 square inches per side of extra side area glued to the underside of the stab of my ARIEL back in the early 90's. That's what that airplane required in my opinion. It looped nicely outside, but in lumpy or crosswind conditions, it would drop a wing on insides. I think you have a yaw-roll couple issue.
1mm Perspex will be invisible at 150m!
Cheers,
Dean Pappas