Update time. "Limited" success to report.
@MM, you're dead right, bits of light fitting here are indeed 10mm ISO Fine. So is my lead screw, BUT my lead screw is a LH thread (light fittings are "normal" RH thread)!
I must say that right from the start when I looked at my lead screw something was trying to "click" in what passes for my brain, but I didn't twig until I tried to screw a bit of light fitting on - I'll be collecting my "D" hat on the way out of this thread! (hammer)
Anyway, no big deal, it just meant removing the "nut" from underneath the cross slide to ensure the thread was protected in the chuck:
As you'll no doubt notice, the nut is not quite square but a bit rectangular in cross section, so it was on with the 4 Jaw independent chuck before straightening could start.
I tried with the hardest piece of hard wood I could find and bored a hole just a bit bigger than the required 8 mm, but being such a short length to straighten, no go at all - the wood gave before the bent part did. So I found a piece of MS strip about 10 mm thick x about 25 mm wide, long enough to give decent leverage and got stuck in.
This did produce some results and I got the bent part fairly straight - enough to slip the ali mounting block off and away anyway.
I then rigged up a pair of vee blocks and used my new "tool" plus a dead blow hammer (hard nylon faced) to get it back to a run out of about 10-11 thou (inches) as per my DTI. The set up was exactly as shown in the 2nd link provided by MM, except that everything I used was metal, not wood as shown in the link.
The reason I stopped at the roughly 10 thou run out was A) my metal "tool" was beginning to "bruise" the handle mount part, and B) the handle mount part had a bit of a banana bend in the middle, which I guess I produced when doing the straightening in the lathe - having such a short length to work with (only 11.5 mm) really made the job more difficult than shown in the links, where in both cases, quite long lengths were involved.
But I've learnt something new and if I ever have a straightening need in future I've got a go to method.
So thanks to all the inputs from you Gents I'm now in the clear, and the cross slide handle is now "near enough" that once the lathe is fully reassembled the handle will turn without being too noticeably out of line.
In between all the above, the motor control box has now been safely packed and sent off to the US for repair. It remains to be seen how soon it arrives back.
Stay healthy everyone, and thanks again (applies "D" hat as he leaves).