this may be a stupid idea, but here we go...
I have no welding kit...yet...
can i just screw 2 nuts on to the remaining thread and then use the inside one with my wrench to "unscrew" it, a little bit like how there is a lock nut and adjusting nut on the rear wheel axle....or is this a crap idea?
As said before, not a crap or stupid idea, but you do have to make sure both nuts are 'locked' onto each other, otherwise it's a waste of time....
First spray with penetrative oil......and leave a while.
Then put one nut as far down as you can get it, the put on a spring washer, then the other nut on top, screw the 2nd nut as far as you can down the shaft, now keeping that in position with a spanner, use another spanner to tighten the first nut you put on, to tighten onto the washer and other nut, once this is done, gently turn the first nut as you would to loosen it, just use the spanner, as puting a socket on it would mean that both nuts have to align and the washer has to be smaller that the circumference on the nuts, good luck