Thanks ever so much for your replies i have found them most useful and can definatly apply some of these tips straight away on my next lesson. Use of the shoulders and making sure my weight is slightly on the front foot is what i shall concentrate on firstly. I can control the board well and the first part of the turn is immediate nose down to the fall line no problem, it just seems like their is an invisible barrier fighting against me to prevent following through. I can assure you i am certainly not bailing early im using pretty much all of the learning slope before i have to pull it back into an edge slip and safely stop. I know its something in my technique that is preventing me from doing this because its the same story every run hit the fall line cant push it through 180 to switch. The more i think about the technique i was using when at the fall line the more i feel confident that i just need to make sure weight is slightly forward and just apply a little shoulder or (naval twist as you put it) and i will be switching edges in no time. I know that if i can just get it to work for me once and feel what is the right combination to make the board push through the fall line i can repeat it no sweat. I tried explaining my situation to my instructor but the advice he delivered didnt work for me again and again so i just felt i had to ask..
Once again thanks for your feedback and tips i will apply it and let you know how i progress in 5 days time after my next lesson.
kind regards dave.