Thanks Noel! I had a look at the ratings on the switch units and they're good to 600V, 10A so should hopefully be fine. I also downrated the fuse in the plug from the supplied 13A to 5A as that should be plenty for that motor and controller.
Thanks to John Hall who discovered how the cross slide nut was mounted during his restoration, turns out it is cap screws from the top. That sorted this weekends project then:
Not only was it painted over, but also skimmed over with filler. A little digging and I was able to remove it.
Two cap screws going through the cross slide, then on the underside a shim and the nut itself.
Got as close as I could with measurements and modelled it up in Fusion. I scaled it vertically to fit in one inch brass stock but if I were doing it again I'd go with the originals height of 28mm – the way I did it has the threaded hole extremely close to the bottom. Also kudos to RDG Tools for having a LH ACME tap that wasn't too expensive.
And there we go. I've gone from a full millimetre of backlash/slop in the cross slide to 0.1mm. It's a little stiff towards the start of travel but I'm hoping that'll wear in.
I also uploaded the vector file I used for the headstock cover gasket, it can be found here