I should have mentioned that about 40% of the steering wheel play is the PowerSteering ram actuation, so when the motor is running the play is much less but still not real good.
I have degreased around the spline shaft & there is a C clip at the top that holds (stops it falling off is about all) the shaft on the spline.

Maybe the arm (female spline) could be slit & blocks with bolt holes welded on to make it clamp the shaft. This would require specialist welding being cast/forged.