I have not yet opened the bearing block (nor do I really want to), but the play is in the outer races of the bearings leading me to suspect that there is something gravely wrong with the unit or something has gone missing in the assembly. If I put on the nut and move the ball screw axially i can see both bearing's outer races move in unison (I will check that they face one another but I am pretty sure that they do)

Should I open it up and if so is there anything I should be wary of?