Solution was a little more than the worm being too tightly engaged.
Problem occurred again today. Ensured the worm was nice and easy fit, but the stepper would not rotate.
Disconnected stepper from adaptor tube, and it ran beautifully. Got a pair of snipe nosed pliers and attempted to turn the section of Oldham coupling attached to the rotary table spindle. It would hardly move, definitely too tight for the stepper to move. The section of the rotary table that engages and disengages is held in place by three grub screws and also has the screw to lock the worm engage/disengage into place. Because the weight of the rotary table was causing it to twist on the mounting, I had tightened the grub screws up. Slackened them off and all moves happily.
Was able to use to then put some graduations on a scale for another part of the Stent.
Not sure how to resolve this. Will dismantle another day, grab a few photos and put them up here and see what suggestions arise.
Colin