Simon,
Similar problems here. I had ordered motors and cnc controller kit from them and when it arrived the motors had been used previously. I was then given delivery dates for replacements which never arrived. I ended up getting a refund after loads of emails and phone calls and a few of the emails were ignored along the way.
My advice is to go for the full refund and then speak to a nice gent called Gary at Zapp Automation. The BK and BF ballnut supports are standard sizes so getting a screw to fit them will not be a problem.
Dont let up on your refund, its your money after all and you didnt get it easily so why should someone else. So keep at it until you get it.

Regards

Ian