Some interesting ideas coming. I have tried keeping in the spin for about 6 turns and trying both directions. Neither time did it exhibit any reluctance to come out of the spin. It just takes a good turn to come out. But the problem is that its not even always a full turn. The best I got was a quarter of a turn. I thought I had cracked it - until the next when it took a complete revolution. I have not changed the CG to check that as an effect because it would effect the trimming.
Using different amounts of elevator and aileron have all been tried as also exiting the spin. The normal model exit type where you just release the controls - usualy gives a consistent number of turns before exit - and also giving opposite rudder but that gives a rather erratic turn count before exit.
I am coming to the horrid conclusion it is the aircraft design, in which case I cant do much about it. That is rather a defeatist view though and I am still striving to find an answer that is both simple, explanatory and that gives a result of a clean spin exit in the correct direction.