411 front axle & bearing torque

[sjp63]

I found that one of my front wheels had excessive movement and thought it was the wheel bearing, however on inspection, the stub axle is loose..!
There is a big stiff-nut on the inner side of the hub assembly holding the stub axle in place (not the bearing nut on the outer side), easily seen with the wheel off. When moving the wheel at 12 and 6 o'clock, the nut can be seen to move, as the sub axle moves.

There is no sign of wear or chaffing.

Does anyone know what size that nut is?

I've got a diagram from the manual, but no size (for a socket) I don't have a socket big enough and need to buy one.

Also for the wheel bearing nut, what is the torque/process for tightening and fitting the cotter pin?

Thanks