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