I found that two 5/64th / 0.078" drill bit shanks put into the nut then held in a vice allowed me to turn the nut but it was a fiddly operation.

The nut came off its full tight position and with difficulty I took it through one turn. I thought that would be enough to at least slacken the assembly of barrel and shroud slightly but it remained as solid as before. I thought it best to turn the nut back, retreat and re-group. I tightened it back as it was.

The internet stripdowns of the gun all show fingers being used to screw the assembly together then the special pin spanner to tighten. Why is mine different? Do I just keep graunching away until it comes apart as suggested by Alakar?

All I wanted to do is knock up a .22 barrel for it - which I don't actually need. Maybe best left alone.