Thanks so much for sharing your thoughts guys, appreciate that. @Muzzer: great link and info thanks. I will try pulling off the nut and see how I go, though it sounds like to get good rapids out of the machine I will ultimately want to change the pitch to something like 15mm and get a smaller toothed pulley for the servo...
Good call on the Place in Korea BTW, I actually got my servos, drivers, and the ballscrews for the y and z axis from that e-bay shop, and have been well pleased. I have looked a few times, and if they had anything in the right length, dia, pitch, I'd be going that route... I will keep checking and might shoot them a message. Though I don't relish the extra cost, I feel the hours I've put into the machine build would justify it...

@Routercnc Thanks for this - an interesting watch... I'll see what I get and how it goes tomorrow. I must admit, my first thought was that the balls might be a bit undersized, or possibly just unevenly sized. I can't detect any backlash (by hand).
Thanks again guys, I'll report back with the findings.