Had another attempt today.
The Nut won again.
Re pin through castellation. Pin is out and hole is clear.
I think friday will see the demise of the nut, we really need to see if we can reuse it but time is passing and we all have better things to do than spend too much time on this.
For the sharp eyed the spanner has been sacrificed to this task, it was 65mm but I ground it out to fit.
The bolt with the washers is just holding the back plate in temporarily. I had to burn those 3 off in the end as well.
I have also spent about 5 hours needle gunning the crap off, I know it doesn't look it. About an inch deep all over in stone dust.