Could anyone in the UK using the Ron Steele plans for a mini lathe conversion tell me what Z-axis lead screw they're using, and where they got it.

The plans quote a high-lead (1/2 inch, I think) Acme leadscrew which allows the machine's original leadscrew, and manual carriage wheel, to still be used by "back-driving" the CNC lead screw.

Marchant Dice have already told me they can't supply such a high lead.
Any other ideas?

BTW, I want to maintain the original lead screw for use in manually-turned spring-making.