Thanks to everyone who made suggestions regarding this problem. I've had her on strong black coffee for a while and she's now as sober as a judge.
The solution - BIGGER RUDDERS - over twice the surface area of the ones on the plan.
I'm guessing there is a formula for calculating the required rudder area for a hull, does anyone know if this is true and, if so, how it might be applied to a scale model?
Scottie