Thank you everyone for all the suggestions and advice.
The lathe belongs to a company I'm working for. While we do have another working unimat and a much larger lathe which is capable of cutting a thread, I think the company would prefer an 'off the shelf' solution rather than someone spending the time making a new screw.
However with the lack of availability of a ready to use replacement and difficulty finding the correct threaded rod, it's looking like a more reasonable option, and it's something I would be happy to learn to do!
The U3 leadscrew looks like a good solution also, and less work involved - thank you Keith.
I will have to offer these solutions to the management and let them make the decision! I will keep you posted
Beck