Any question answered...

So the one I linked to, the fixed thread and lockring thread are one unit that just screws onto the freewheel thread? That sounds a bit crap for £80.

It would be a bit crap for half that. The only thing is solves is chainline on wheels which can't be re-dished.

And would it be possible to find someone kindly to do that for me, and use their local machinist instead...?

If the wheel has 12mm internal diameter cartridge bearings both sides, you can use Surly Fixxer parts, no custom fabrication needed.