Well checking the FEDEX tracking number Chai gave me and my ball screws are in France at the moment!
Remembering Kingcreaky's build thread where he got different ball nut housings than he thought he was getting I asked Chai for the data sheet. Good job I did as they are different from what I thought I'd be getting (best ask before ordering next time just to make sure!) so I'd best re-draw them and adjust the mounting points accordingly...