Hi Nick,
I started experimenting with a one plane swing recently and found out, a bit like you, that It was near to my current swing anyway.
I have thought about and studied the swing plane and consider the nearer you can get to maintaining the plane set at address through the swing, the easier it will be to return to it at impact. The one plane swing can be imagined as a circle laying back on the angle of the club at address.
I imagine a line that passes through the ball directly from the target then continues on behind me. The takeaway should follow the circle with no rolling of the wrists or forearms. Halfway through the backswing I want the butt of the shaft to be pointing at the ball. Throughout the rest of the backswing the butt should continue to trace along the line.
The transition between the backswing and downswing will necessitate a slight lowering of the plane (still parallel) as the legs and torso sit down into the shot, the club should retrace along the same line to the release position.
I now use this method with all of my
clubs and like the simple way of achieving consistent solid contact.
I hope this is helpful and clear enough.