thanks guys for the help, I had a real close look at the leadscrew and yes its an acme. I set about cutting bronze replacements using believe it or not the worn out half nuts. The lathe was not running, I made a handle to screw into the drive pulley and left the half nuts engaged and commenced screwcutting by hand.
Went a bit deeper and wider on the thread than I thought and when I finished it I fitted it to leadscrew, not to bad, still tight, I'll put a bit of lapping paste on it and lap it in on the far right hand side of leadscrew. this area won't be used.
I also priced a new set of half nuts from hercus and they are $A168.00 plus delivery. So Ii thought I would give it a go to make my own.
I'll keep you informed on how I progress kind regards Twistdrill