Finally I got around to having another go at this part. Now with the compound slide removed and replaced with a raising block things should be a lot more rigid.
The crosslide gibs nipped up and I added a temporary DRO. This really made a difference as I could retract the tool before winding back the carriage and re-position the tool exactly where I wanted it. The DRO was only temporarily held in place with some clamps but worked perfectly.
And this is the finished article. After all the effort to get this far and considering the lathe I'm using I'm chuffed to bits. about 1" 6tpi thread. The tool was plunged in, if you can call 0.10mm per pass a plunge, reducing to 0.05mm towards the final cuts. The lathe coped fine, a touch of flexing but not enough to worry about.
That should do the job