Malcolm,
It's good to hear the small winglets solved your problem. You might want to check these posts too:
http://www.rcuniverse.com/forum/elec...l#post11194747
http://www.rcuniverse.com/forum/elec...l#post11194764
http://www.rcuniverse.com/forum/elec...l#post11202318
Quotes from the posts above:
"I modified my wings with winglets like the Wind S Pro GFK version the bubble tips these seem to work better than the flat ones used on the Oxai Citrin. I also have them on the tips of the stabilizer for rock solid flight in windy conditions."
"I personally think that it has something to do with the aerodynamics since the bubble version is smoother compared to the regular thin fins wich are noticable nervous during turbulent weather. I tried different styles with the thin fins, toe in and out and straight. Also experimented with height and dept. Because I fly a bipe it is more work and harder to notice the differences.
The bubbles on the stabilizer give me a big difference in loops the tail of the plane tracks better."
I hope this helps.
Alex