Ok, I got to the bottom if it.
The reason my volume wasn’t showing in /dev/zvol was because the block had a volmode of none. It needs to be dev to show up as a device.
sudo /usr/sbin/zfs set volmode=dev Storage/.ix-virt/custom/default_Windows10_Office
Now I can see it and dd.
The virtual machine booted immediately on my desktop and I could retrieve the things I needed.
This evening has been a learning experience 