Doxford,
I recently suffered from the same problem, a distorted tooth on the camplate spindle gear. Try the following:
Remove the inner cover and the layshaft and all removable gears, leaving only the sleeve gear and 4th lay. Lower the index plunger until it still indicates but does not interfere with the motion of the cam plate. Remove the camplate spindle bolt on the outside of the gearcase and slide the camplate away from the quadrant until it rests against the sleeve gear. The camplate and quadrant can now be independently rotated on their respective spindles but CANNOT BE REMOVED due to the interference with the sleeve gear. You can exercise both pieces sufficiently to be able to tell which part is preventing smooth meshing, and whether these two parts are the ones causing your problem. While diagnosis is straightfoward, repair is not. In order to replace the camplate and/or quadrant, the sleeve gear must be removed, which means the sprocket nut must be removed, which means the inner primary cover must be removed, which means the outer primary cover and ALL of the primary components must be removed. You will wind up having to stip down the entire drive train excepting the engine itself. The VERY small consolation in all of this is that the camplate/spindle can be had for $60 and is widely carried. The quadrant is a bit more and only a few suppliers (Old Britts, e.g.) carry it. I understand that some brit marques foresaw this problem and cut away the backside of the camplate, allowing for its removal. Alas, not Norton.
If there is anyone out there in Norton-land who can think of some other method of replacing the camplate, let us know. This is a gruesome job.