that's fobarred the ball has split and ridden up the metal shank and the bottom of the metal part is probably sticking out the bottom and fouling in the cup
should be a decent fit on the splines and well secured with thick washer and nylock nut and no play before take up of movement
new genuine from Brickwerks, cheap chinese off ebay, cheap ones don't fit well in my experience I'd rather fit a new genuine ball to the old metal part if ok but if yours has got worn splines then no point refurbing it
clean out the cup, check it's not damaged or excessively worn and relube and fit a new skirted boot 251 711 167E to the whole to keep it sweeet
