RE: How to avoid "spin outs"?
After you sail the boards with the "cut off to 33 cm" fins in Bonaire, NA
(if you sail in Lac Bay at "The Windsurf Place" or Jibe City) you may find that the fins on your boards at home in the UK are "rock solid" in the spring.
When will you be in Bonaire?
Sorry I'm going to miss you as I'd like to see what your stance is and how you are sailing to get a better idea if your "fin issues" are totally related to the fins, or if there are some technique issues mixed in there as well.
Enjoy Bonaire! B)