Hi Ian,

You're missing a key bit of information - how are the screws retained axially?

Typically people use a fixed block at one end (often the driven end) and a floating block at the other. These are often of the FK/FF or BK/BF variety.

You might be able to feel the screw moving backwards and forwards as you pull on it with 0.15mm of backlash, if so that would imply you need to look at the screw rather than the ball nut.