I know this is an old post but I am having similar problems. Although the parts book stipulates 11 plate for all models, the friction plates listed for the P11 , 043192, are 4.5mm thick and the stack would be 35.3mm thick and the basket is only 32mm deep. RGM sell the 043192 plate, they also sell a thinner one at 3.5mm thick making the stack 30.3mm thick, so far so good. The pushrod from the parts book, 040607, listed for all models is too short, the commando pushrod is longer, however with the springs under tension it is trying to push the whole clutch out. The bike is a1966 N15. When I rebuilt it I ran short of money so cleaned the clutch and didn't put any oil in the chain case. It worked ok but started slipping so, having a bit more cash, I decided to replace it. It was a 9 plate clutch, but the pressure plate, one side plain, the other with pads was 5.75mm thick, not 2.8, this increased the stack thickness to 30mm.
So the parts I have should work, I just cannot get it to actuate correctly, any ideas? Anyone replacing a clutch should be aware of buying 5X 043192 plates as it will be too thick.